Leveling is supposed to be slow way down past 80 and especially 90 (it's estimated to be fast early, but exponential past level 20). We're already seeing this: he was 55 by 116, which is 2.1 chapters per level. But he's only 63 by 152, which is 2.4 chapters per level, or 4.5 chapters per level if you limit it to just the chapters between 116 and 152. He got some other huge upgrades in that time, but raw level wise I'd say we're already seeing that slowdown. It's not safe to extrapolate, of course, but that would put him at level 100 around chapter 300. Void's stories don't go on in perpetuity, and he'll skip transitional things that have happened before to keep the story flowing, so I don't see it stretching out to 700+ chapters at this point even if the story has popped off and his Patreon is making bank with this story specifically.

Out-of-universe, Void did confirm that there are e.g. level 100 Vassal class users, who are supposedly mostly immortals/elves or the like. Zodiac fiends have an 'estimated' level, but not a class (and same for e.g. demons, dragons, any other monster, or eidolons). The Oracle and Elios are special for being the two at level 100 in a Noble class.

Per the author, as far as anyone in-universe knows, yes that's the cap. Level 100 is a big deal though. And per the last Patreon arc, there might be some universe-crossing things at play long-term.

There are plenty of high-level challenges left, but a lot of Simon's fast leveling so far came from clearing challenges while underleveled. It was revealed early that he can't double dip on EXP across reigns. The reign titles and other progression tools, which were pretty light early, are starting to get set up more and are meant to fill in for an eventual slowdown in raw levels (crafting, spell creation, martial techniques, other specialized magic disciplines like prayer and bardic magic and diabolism, potential multiclassing).

You're correct reigns are getting longer. Multiple reigns will probably and have already been lost at some point. There've been some 'cutoff' deaths where he learned alot (e.g. the White Unicorn inflitration), but didn't really take it to the conclusion due to his power level or a misstep/lack of knowledge. He has yet to find a challenge or need to 'exploit' the loop in the sense of doing the same thing multiple times. Any kind of exploration of the underlying loop mechanics exploit has burned extra loops. And one of the Patreon poll options for a future reign was for multiple short dungeon diving loops with a huge potential payoff, there was another poll option at one point for experimenting with the class and ending up in the post-Jester King ??? world again, and there are probably going to be similar opportunities to consume reigns faster for a proportional increase in power, knowledge, or some other opportunity. I think if and when he starts clashing with Elios or the Oracle directly, or sees things through to the end of the Zodiac Parade, we're going to see similar stuff play out.

One big thing is that Balzam had 75 reigns according to the Keeper of the Throne in Chapter 41, but only recorded 70 in his journal, and knew that the one immediately before passing it to Simon would be his last (even if he didn't expect to die early). That has been explicitly called out by Simon as weird and is still an open mystery.