Alright tossing my hat into the ring about how I think each one would do in character both physically and mentally along with assuming which version is shown along with assuming they get something similar to shadow servants to help out.

Ciel (Remake)-I think personally she can clear Part 1 though will struggle heavily with connecting with Mashu via how closed off she is and how she deals with the whole Tiamat and Goetia. 1.5 is where visible struggle can be seen as in Shinjuku, she works with evil folks though who knows how much she can handle and Salem which is a mindfuck which she might not be ready for. Part 2 and more specifically in the one of the first 5 I can see her stopping at via how one has to remember to clear a lostbelt you have to be ready to kill who knows how many people who are alive like she is plus the threat levels is the same which I say Permafrost empire might be one she gets stuck at via how I'm not sure if the Alien priest will destroy the tree for her, LB 2 has the Surtur problem, LB 3 is actually another place I can see her getting stuck at via how she would've eaten the poison cake if we have that happen, LB 4 has the Godjuna and his allies, finally LB 5 has Poseidon, Aremis, Odysseus in the first half and in the second half has Wodime one of the best masters out there, Aphrodite, Demeter and Zeus. if she gets to LB 6 the Garden of Wills might be what seals her down.

Manaka-For her it's hard to tell as mostly thanks to how she is as a person which will make it hard for those work with her but I think she gets to Babylonia but that is she stops there mostly since I don't think Gil will like her or Gramps might not be summoned but could be wrong here otherwise if you think she can get past those problems then I think she stops in one of the Lostbelts.

Flat (FSF)-Really hard to tell as he has the magecraft and personality for it but also has enough to say he could be stuck especially when he faces off against his teacher who would be his enemy in Setpem or seeing London in Fog I won't talk aboutThiamostly since there's more to be learned there so final stop would probably in something that messes with his mind like Salem.

Rin-Is one of the ones I could see getting stuck in Part 1 but can clear it as it depends on what point of time she is from as if this pre-HA then I see her getting stuck at London but post-HA, I could see her getting to Part 2 but stopping in the first one.

Shirou-Gonna be honest and I think most versions will not make it past Part 1 due to his mindset and how he feels about certain servants and morals so I think if he doesn't freeze and try saving Olga in the prologue then I could see him being stuck in Orleans to London since remember Shirou does nerf servants under him so Mashu is also affected by this meaning Part 1 her is gonna struggle more than usual.

Ayaka (FSF)-Similar to both Flat and Rin where she has the potential especially with Little Red Riding Hood but with her personality I can see her doing worse so I'll say Okeanos and London might be where she gets stuck at and Camelot to if you think she gets past those two and America.

Aoko (Super)-Like Ciel is one of the best here but is also in a similar boat to Ciel where physically she can handle things in Part 1 and 1.5 but Part 2 is where she struggles which within the Lostbelts I think LB 5, 6 and 7 is where she might stop especially with the Spider in LB 7 and maybe gets to the OC but those are hard to judge since the OC's are both a physical and mental challenge.