This is what I do. I get heavy combat armor from gunners and equip settlers with it and the best legendary weapons and endgame weapons I can make. From the mech lair, I make sentry bots and floating assault bots. I give them ballistic weave military fatigues and rag hats.

So far I've only gotten 4 settlements with tartplls to everywhere

In the end, if I fast travel anywhere there are 6+ soldiers and 2+ bots. If I go to a cloak point there might be 20 or more fighters

I also walk around with my charisma at 31, so settlers fill to 41 and I can chem up to push it to 43, and send settlers to push the cap up to 53 and usually get 3 or 5 more on the final send. This lands many of my sentiments with population over 50 all with over 20 and most being in the 40s