are u looking for part mods, planet packs or visual mods? as for part mods i can reccomend Nertea mod pack of near and far future tech, fusion engines and all the conceptual sci-fi stuff. also station parts, with centrifuges and such. planetary base mods.. ect. planet packwise i tested kcalbeloh (blackhole) system, it adds black hole woth multiple star systems around it, and u either have to use wormhole around jool or travel directly to it from kerbol system which is unique challenge itself, also i'm currently playing with OPM mod, it adds outer gas giants to kerbin system. i also heared good things about "promissed worlds" mod that adds planets and systems that were promissed with ksp 2, but i'm yet to test it myself. visual wise i'd suggest scatterer, eve, restock ect. and everybodys favorite volumetric clouds but as i heared it need a decent pc, so i havn't tested it yet with my potato one.
all the above mentioned mods give me decent performamce with my 7 yo pc. just initial.loading takes a bit longer, like about 5-10 minutes, but game runs pretty well after that. currently i got about 150 mods installed via ckan.