Build a strong balanced army and advance slowly, deploying the right counters.
Infiltrator raids are absolutely still a thing but they aren't supposed to take defended worlds, they are about hit and run attacks.

If you want to sandbox to get used to mechanics and try things out I'd recommend a table for cheat engine.

Turtle, using the "hunker down" button option or whatever it is called for infantry, then advance when incoming attacks thin out. Use healing units. Use 1-3 artillery which will delete most of incoming units. Air units are also good to deal with incoming infantry waves. Use some fast unit (I usually use a Jedi) to scout and then use bombardment/bombing runs on large concentrations of enemies when spotted.

This mod is actually easy for ground battles, e.g. in TR you mostly get incoming massive attacks early on which I rarely see in AOTR 2.10 even on highest difficulty.

But best tip would be to watch some ongoing YT playthroughs e.g. for rebelz look at XPGamers ongoing AOTR 2.10 campaign, for Imps look at the GenesisKing ongoing campaign.