Rank 2888 here, 272124 aph with 493 planets occupied; here is my feedback:
Planets always worth invading:
Any VM or bigger arti planet (best kind of planets to terraform)
Planets with extractable cloaking artis (CSR / Ergo / Meta / LCT / etc)
Good arti planets with inactive owners (350+ aph)
Planets with GP stuff (warehouses / scruuge vaults / etc).
Once invaded, if I don't want to keep the planet I use research optimizers to make it 15x and leave it undefended; most of the time is taken back if the owner was active (else at least is producing extra credits).
I do not colonize planets anymore, but just because I'm waiting until I get my 3rd year reward in December (then I will colonize 100 planets in one go if all goes as planned).
As a general rule for planets:
Research:17x colossal plasmas only, leave undefended
Mining: 16x / 15x VL toxics only, leave undefended
Artifact ("meh"): ~220 aph, up to ~400 aph, leave undefended
Artifact ("great"): ~700+ aph production, at least 18k cloak
Storage: use the colossal plasmas from above once you don't need research; easy to cloak to 18k+ using satellites and a polaron emitter - 5 arti warehouses and a poly vault only on each one (very cheap to make, and if lost - which is next to impossible with so much cloak - it's just 4k ctp).
The arti planets in between "meh" and "great" (e.g. an average 9x arti exotic), I usually focus on production but I probe them from time to time.
Don't invade any mining or research planets unless they have something from the first list, and avoid making too many.