Well, I'm a coaster veteran (ridden about 650 coasters all over the world...), I don't get that "sick" feeling, so I'm just trying to understand what it is you like and don't like! Now, Tower of Terror would certainly be a ride that I thought would give you that stomach in mouth sensation, which is similar to a coaster with floating airtime. But if you like Tower of Terror anyway, then maybe you might like Incredicoaster. Perhaps someone else in this topic hit the nail on the head when they said that the big drop on Splash Mountain is slow. I can see that - you start off from practically still at the top then it feels like the boat is just slipping away from you that you have no control over. That's the same feeling you might get towards the back in a coaster. But at the end of the day, Splash Mountain is just a log flume.
When all is said and done, I always say you might as well try it once. If you don't like it, then that's fine, you don't have to do it again. But you might just find something special.
Yes, WDW Space Mountain can be brutal! It too has some airtime moments but they're more abrupt. It's the same ride system as Matterhorn Boblseds in California. But California Space Mountain is a completely different system, layout, and is much smoother.
Also, when comparing them with Rock n Roller Coaster, if you didn't know this before then go searching on
Youtube for a ride called XPress at Walibi Holland. It's exactly the same as Rock n Roller Coaster but out in the open, so you can see what it does and compare it with Incredicoaster (spoiler: they're nothing like each other!). And these rides are similar to Paris Space Mountain.
Big tip - try not to stiffen up on these rides. Especially coasters. You said that you brace yourself, and of course for safety reasons you have to make sure you're stable and ready for any jolts, especially braking. However, on a coaster if you tighten your muscles and grip really hard, you're going to feel every bump through your body and it's going to be jarring. Try to "go with the flow" and just move wherever it takes you, while still being aware that it might throw you around a bit. Try it next time you're a passenger in a car - really dig your feet into the foot well and grip onto the door handles tightly - you're not going to find the ride very pleasant!