Moats are dug with the pick, no need for a shovel.

I had the same raid hit me right after I beat the first boss as well. I went outside and killed them all. But NOT by trying to charge into their midst and tank all of them at once.

I had already planted a LOT of trees around my base, so my strategy was to go into the trees and lure them to come after me. Because of all the trees they didn't all surround me at once and could not hit me with ranged attacks (trees provide cover). I was able to finish them off a few at a time, by always staying mobile, attacking the first few to reach me amid the trees, moving again, maintaining cover, rinse repeat.

As for avoiding being surrounded just trying to get outside in the first place, my base had a back door that is not accessible to mobs (it's elevated), so none of them were there. That's how I got out and into the woods in the first place.

I'm now near the end of what we currently have for the game (only one boss left) and have NEVER built a moat. The game is not at all 'impossible', but the raids are not to be trifled with, you need to have some kind of defensive plan in place and you also need to make sure you don't think the only way to approach them is to 'face tank' the entire raiding army at once.