i tried both 30fps mod and quality mod. The 30 fps introduce framepacing issues, in loadings, movements, even with max oc... its much less enjoyable than without and its not a performance issue, some square games like ffxv had the same issue on pc actually once a forced third party 30 fps cap was in place. And coincidentally also, if i remember well, the game is not capped at 30 fps, but rather 31/34 fps with the in game official fps cap. The quality mod, even with max oc on both cpu and gpu, has heavy slowdowns sometimes, and some bosses are near unplayable (the second ice boss for exemple).
The issue to me lies in both the switch power (for the 900p mode) and square enix port of the game for the 30 fps cap mode. there is something very wrong in how the game handles fps, the animation of flammy is very choppy and weird without any cap, and becomes smooth once the cap mod is in place. Unfortunately, the game seems to rely on the uneven fps, as loadings becomes very choppy and gameplay has a lot of framepacing issues once the modded fps cap is in place.
i don't think there is anything we can do as of right now to truly improve the game. I did not try the 60 fps mod but i think it may be the only one truly improving gameplay since this game seems to want a higher fps count, but the quality price is too high for me imo. To really improve things, that would require a better switch for that.
on another note, i noticed oc the cpu without any mod whatsoever improved a lot the fps when there is a lot of enemies on screen, when oc the gpu only marginally improved that situation. With both max oc on gpu and cpu, the game rarely slowdowns while everytime there was too much enemies on screen i would get some slowdowns on stock everything while docked. Dragon Quest XI S didnt really slowdown for me, but i also noticed an improvement overall on dynamic resolution when both of them were OC to the max vs only gpu. It seems they share something in common, maybe UE4 ported games are very cpu hungry on switch, much more so than real switch exclusives like nintendo titles and benefit a lot from cpu oc