Your writing is really nice to read. Ren's strengths and weaknesses in character are highlighted in ways most fanfics can't pull off. For that, I feel obligated to share with you a few details, specifically about the Persona side of things.

For starters, Persona constitution and defense: contrary to what it might seem, the in-game HP bar IS actually an accurate representation to how Personas defend their users. With the user's life force and stamina, a Persona forms a protective aura which shields the user from all manners of assault — effectively acting as HP, so to speak.

This is why Persona users don't suffer injuries or even clothing damage unless they are worn down to the point their auras can no longer defend their bodies, or if they are attacked when their Personas aren't present at all (note Personas don't have to be manifested completely for their auras to defend the users; even those iconic blue flames being present are sufficient), at which point they become as vulnerable to injuries and death as any normal human.

Related to this, healing spells do not actually heal wounds, when outside realms constructed in the collective unconscious (they do heal injuries when inside realms like the Metaverse due to laws of cognition); they instead replenish the life force empowering the Persona's aura.

This is why healing spells don't work on humans who have yet to awaken their Personas (shown in Persona 1), why actual body injuries still require regular medical treatment to heal (Persona 2 and 3), and why Dia spells can be used on robots like Aigis even though mechanical bodies are unaffected by healing (because it actually restores the Persona's energy, that being the life force).

Basically, as long as a Persona user still has stamina left, and they are not dropping their guard down, then injuring them is virtually impossible without depleting their aura first. This means Ren and Morgana will become increasingly difficult to kill, even for Servants, as they slowly rebuild their strengths back to their peak.

Of course, there are means to bypass a Persona's aura, in the series itself it is shown high level conceptual attacks such as the Spear of Longinus used by Nyarlathotep in Persona 2, or using cognition to overwrite reality like Yaldabaoth nearly wiping the Phantom Thieves from existence, can bypass this.

However, by Nasuverse standards these would be very high ranking Noble Phantasms, by all accounts. Most Servants wouldn't be able to nullify a Persona's defense, and would have to overpower it with brute force . Of course, seeing as Joker and Mona are both far from their peak at the moment, they are not as durable as they used to be in their Journeys.

These should have interesting applications in your story: healing spells might not work on flesh and blood injuries when in reality, but it would work on Servants just fine (due to their bodies actually made out of Ether, they are constructs closer to energy beings like Shadows).

Similarly, the concept of using life force as something akin to a force field barrier is a concept certain Eastern Servants like Li Shuwen would be very familiar of.

That's all I want to say. I hope these would prove useful to you. Keep up the good work.

Yusuke might have like Da Vinci wouldn't he?
Considering how Timey whiney the Throne of Heroes summoning is with people like Emiya... if people still remembered the Thieves, I can definitely imagine some public idealization imaginative cognition of the Phantom Thieves being summon able.
They would represent the silhouette but without more details they wouldn't likely be the Thieves we know.

Has me really wondering where Joker's and Morgana's weapons are with mention of weapons again.

Hey, is Strikers canon in this story too?

Perhaps rather than letting Olga run separate from Ren he should use himself as a means to help sustain her.
Though Lockdowns on "Personas" can make them stronger... The Lockdowns on Persona's are also dangerous to their existence should they be separate from their user for a long time too.
Let's hope Olga gets stronger then... let's light some incense for her.

Unfortunately I bet this Cu Culainn doesn't come with Physical Null?

So the Persona system is piggybacking off of the Thone of heroes huh?
I wonder if the Sea of souls will be able to manage a partial repair as things get better... the potential of both has their own strengths.

Guess it didn't sink in until now that everyone outside of Chaldea died.I wonder if Joker is single too.

Speaking of the friends and past... will there be any mention of that Grail the Thieves saw and fought?
Those cups are supposed to be a big deal after all.

Joker's going to really wish and want a Physical resistant persona huh?

So what's to happen Olga? Is she to take slot up in Joker's now likely vacant persona pool? There's plenty of room in there after all.
If she can't sit in the Velvet room forever Joker certainly has room for her in his heart. Hahaha.

What about his weapons that were tied to his Thief outfit? Morgana's as well.
Considering the material and personas that could go into those , it may be a little equivalent to carrying an interesting weapon. Not many weapons are thought to have the power of Satan in them.

I hope Olga isn't just forgotten in the background forever at least. Even if nothing is to be done now about her, I imagine later servants might have an idea?

Ren blinked. "Ah, no, just…" He thought back to the Cu Chulain he knew back from his adventures. There were definitely a lot of differences between that one and the one standing in front of him, but one of the biggest ones was… "I thought you would be wielding a spear or something."
Xxx
So he remembers the Persona. What would it take to get it back however?
Or at least a new one.

Honestly with weirdness around Ren, I wouldn't be surprised if another got a persona though it'd definitely be limited to a very few people.
The servants are out of the question most likely.
Leaving really only Mash and Olga if at all.

So more are other personas are likely to be able to be brought back later as Chaldrea makes progress perhaps? They will be interacting with most of the legends that Personas are based upon like Cu Chulainn and Scáthach and the like.

As history is slowly fixed so too their place in the Sea of Souls?

You have grasped well the persinality of the characters. Their replics sound believable and I can imaging them saying these things. Well done!

If only they knew that the strenght of Wild Cards like Joker came from their bonds with other's.
Not saying they are nothing without them but for somebody like Joker those bonds mean the world to him, and now those bonds are gone.

I really liked the interaction between Joker and EMIYA. I also really hadn't expected Cu to point that out but that would be expected if they wanted to ensure the safety of humanity. I also do think that there should be more interaction with the general Chaldea staff like Sion (but then again I'm not sure if she's around now in this point of time) or even Maomao but that's probably just me. Also, poor Mash keeps getting more questions whilst also being offhandedly being shrugged off.

bro this has a future, the relationship of all the characters, I hope to see how ren is surprised to use cu as a person

Reactions to Da Vinci were on point, shocked, and a look of "what" at Da Vinci's explanation for her form.

Ren not recognizing Archer as the shadow servant he faced, well it was a shadow version of Archer Emiya.

The Cu joke everybody! Also, where is Caster Cu, he comes back with you after Fuyuki, not gonna add that gameplay detail of welfare servants.

Again, Can't wait for Ren's reaction to the gender-bent historical figures, Especially Yoshitsune!

EMIYA and Cu, did Ren and Mona pull during an event banner for the 5th HGW?

Maybe if they're lucky they can pull Baeber, Rider, Second Saber, Ber-Zer-Car, or Backstab Witch.

Might want to stay far away from summoning Gilgamesh.

Also, I think it's funny the Cat/Not-Cat has a mutt for a Servant.

I a

marie can come back, unregistered saint graph for soul container and fate verse has homonculi clones and doll makers who can make a body for her,
also imagine huge puffball fou after all that eating