I have no idea where other people may be up to in this mod.

I'm wondering whether I'm trailblazing here, or people have completed the progression, and if there were severe balance issues then these would have come up already.

I'm thinking that the SE sciences were more expensive because of the chains which go into them - as expected. No surprises there tbh. The fusion science and matter science are kinda too expensive imo, because they chew on stuff that they arguably shouldn't. But abusing the ability to make broad fusion science on the ground with the much cheaper recipe it isn't anything I would whinge over.

But, the exotic science.... Wow, that stuff is way, way, way too overpriced. It might be bearable with base game rocket launch costs (though I question even that). It's not just the resource costs either. I chose 1000h ago to go with a hybrid base which is mostly belt based, and 90 items (x multiple lanes) per second will bottleneck me on circuits for making this. A rail base would have bottlenecked me because essential components have stack sizes of 1. And I even considered pumping molten metal to small independent bases which would remake basic components and rail out finished products. The 2.0 fluid system is not up to this.

So, question is simple- am I the first one here, finding this brick wall of factorio's limits and having to set up like 1SPM when I've been aiming for more like 100SPM up till now? Or have others done this and decided "this is fine"?

I don't know for certain. There were some people who were pretty far in 1.1, but you might be the leading edge for 2.0 players... or you could just be more talkative than most.

But abusing the ability to make broad fusion science on the ground with the much cheaper recipe it isn't anything I would whinge over.

FYI, that's probably going to go away at some point in the future. Those two recipes originally made different items, and it's really not clear why they ended up conflated the way they did.

But, the exotic science.... Wow, that stuff is way, way, way too overpriced.

Exotic's pretty damn wacky for sure. Though, on the other hand, unless you're getting into infinite techs it isn't actually used for a great deal other than the "make the 248k element" process. Perhaps the idea is to just have that grinding out slowly in the background while doing DSS 2-4? I don't know if that works out in practice though; my own playthrough is nowhere near that point (and basically stalled anyway).