40fps at 120hz would probably feel worse than at 80hz, there’s something magical that happens when your fps are exactly half the refresh. Everything else doesn’t seem to line up as well and you get judders. Although I’ve never tried this exact combination, just a guess.
60fps at 90hz is also terrible. If I were you I’d try to tweak settings to get 45fps at 90hz or upgrade and try to get to 72hz, which is amazing (but not too easy to get here).
And yeah, RTSS seems to be the best frame capper for this use case. The Aero’s “sync to half refresh rate” in the Base software is also great, that’s what I’ve been using, after using RTSS for a long time.