Mods for Low Pcs

It all depends on what mods you wanna use and what version of Minecraft you're playing on. There are fantasy mods, tech mods, mods that add new dimensions, etc. From technical mods, I recommend Create, or Immersive Engineering. I believe they're not particularly resource-intensive, and they do add some interesting machines. From fantasy and magic mods - Thaumcraft or Vampirism - Become a vampire! will definitely work, unless you're not interested in such mods, of course. As for mods that add new dimensions, I'm not too sure what I could recommend, mainly because most of these mods require a lot of resources to keep the server from lagging.
I can also advise you to look for mods in such a way, so that they don't all work at once. If you add too many mods that add new mobs, chances are that the Low End PC won't handle it. If, on the other hand, your server has, for example, Decocraft installed, it should not eat up resources until one of you puts a piece of furniture in your base, therefore reducing the FPS drops. Forgive me for such a long answer, but somehow it just turned out like this.
Have fun.

Hello, try mods with skyblocks like sky factorio, project o zone, agrarian skies.

Maybe sky factory can be good for you.

I know skyblock map is good for slow pc, you start with less then 50 blocks

try playing a skyblock map / modpack since it doesnt really need to load alot of blocks

Theres a bunch of mods. You can probably look for mods that support low-end PCs