so maybe some multiplier to their spawn rate would be nice
Moar wrecks for our scavengers!
The suggestion you proposed for solving the "invaders too easy" problem will fail to work, at least to work properly. There will be just more ships with same weak spots, and good enough commander can prepare for them no matter how large the invasion is. Or not, depending on tech and industrial potential: what can you do against single invader ship when you only have ion engines?
Either too hard, or too easy.
AMM launcher ships? Counter with a lot of armour.
ASM launcher ships? They are pathetic, a PD system which is 2-3 generations behind should be able to shoot missiles out, and then it's free minerals. And RP.
PD laser ships? Hmm, use the range to your advantage.
AS laser ships? Missiles, don't hesitate to launch them.
And that's all I can remember. That's it, four types of ships, each has its own counter.
The problem still remains. They are either too easy, in your case, or too hard, if you look from perspective of lower-tech empires.
I don't want them to be "too easy or too hard".
I want them to have proper fleets, with good enough doctrine.
I want them to have proper missiles, not the current "horrible, but long-ranged" stuff. Hm, I want them to have both long-ranged and short-ranged missiles.
I want them to use everything they can get with their tech level. Cloaks, shields (even standard will do, but absorption will do even better), fighters, HPMs, mines, mesons, FACs, MIRVs, ADVANCED weapons.
At last, I want proper ship selection on spawn. Had a laugh upon seeing 15 gate builders from one wormhole.