Same thing for me. I've given up on settlements, so it's Red Rocket alone with dogmeat. Best place, plus it has all the crafting stations already there so no need for charisma at 6.

And even dogmeat's doghouse is far from my living quarters. I only keep it for guard dog duties.

Settlements are never attacked, when they have no settlers. Somebody on Reddit found this in the scripts. I also use Red Rocket, like most people here, for me alone. There is no food, no water and just one bed. There are no settlers, not even Dogmeat.

I also get the attacks you describe. I think, these are not 'settlement attacks', this is just a normal patrol (mostly Supermutants, can also be some Raiders with low level). They come into range, and attack. This clearly only happens, when I am there. Hence no attack message, it is no settlement attack, it is just an enemy patrol. I also see them near Abernathy, but their route is far enough so they do not attack, except when I attack. If the cell is unloaded (player is not near), these patrols pass without effect, there can be no combat in an unloaded cell.

I also have a special case of enemy patrols in this area: 3-4 Supermutants (including a Suicider, with higher levels) and one settler. The settler is not missing in a settlement, it is an extra NPC. If you kill the Supermutants, the settler stands still. If you talk to him he spouts some usual nonsense, but does not mention you saved her. After talking the settler starts to run toward the nearest settlement, Sanctuary in this case. If he reaches it, he stand there, and sometimes can even be assigned. A short time later you have on settler more. Sometimes he will not stay, and disappear when you leave the area.