You don’t need to use the mod manager, you can manually install it. You are also playing a Bethesda game, you are already playing Russian roulette with stability.

There is probably a mod that will re-enable achievements, if not yet, there will be soon.

Edit

It looks like this may do it, though I haven’t tested it as I don’t have the game. https://www.nexusmods.com/oblivionremastered/mods/125?tab=posts&BH=2