Oh that's hilarious. I thought this was a 3d render made using one of these programs because the tabletop it's on just looked black to me. I tried using one of them once for planning a dungeon and it was such a pain in the ass I stopped.
I use cheap base plates from amazon and do every peg equals 2.5 feet. So 5 foot squared would be a 2x2 peg square like you said. Take note that since you’re using minifigs (and not microfigs) this scale doesn’t really work well 1:1 with the official maps if you’re going to build using their grid scale (battles and exploration tend to get jammed up really badly because walls tend to be a bit too close). However if you’re like me and relatively loose with enforcing distances, it actually works pretty well for battles and gives you some creative license with your builds.
Before I got a bunch of minifigs I was building micro scale and used every peg equals 5 feet. That worked decently as well.
I exclusively use legos for player minis and I try to use them as much as possible for NPCs and Monsters. As far as dungeons and taverns I use Chessex mat. I don't feel like you are ever going to get the level of flexibility you need with legos. Sometimes I will add in lego sets (like the one below) for flavor but they are always used as an addition to the battle mat.
https://www.amazon.com/Chessex-Role-Playing-Play-Mat/dp/B0015IQO2O