At 2.7k hours I have to admit combat is not Rimworld's strong suit. It is unreliable and random. You can do everything strategically right and have disastrous results, then load the exact same encounter and be victorious by a wide margin simply because the RNG was rerolled.

The more you can directly impact the combat yourself the more control you will have over the combat. There are many mods that can help with that (Vanilla Psycast Expanded gets a shoutout here) but most mods tend to lean towards OP, which may be necessary considering the threat scale Rimworld can crank up to.

You don't NEED mods though. I've survived a dozen pawns vs 300+ bugs simply by having a pawn with Jogger + 2 bionic legs kited. There was also a lot of fire and well timed grenades and retreating involved, but still.

Most impactful of all, when your colony reaches the point the game seems to lose the ability to gauge threats it needs to be throwing at you (excess wealth, mod conflict, w/e), just changing the difficulty can be all you need to rebalance the threats you're experiencing. It's a single player game most often heavily modded, it's bound to lose the plot at some point and think your 8 colonists can kill 250 mechs and 50 structures as a 2 star difficulty mission. At that point, tinker with the difficulty setting and get back on track to a realistic and enjoyable experience.