Glad you enjoyed it! I've not open sourced this game, but I have covered my tactical architecture pretty in depth on my site and YouTube channel if that's helpful. Doesn't always go into specific code details but does go pretty in-depth about how I've structured things:
Sure, I've used both of those, plus some others, but the main reason I develop in Godot is that it just "clicked" for me right away, whereas I find the typical Unity workflow less intuitive, but that's a highly subjective thing. Unreal was out because I work in 2D way more than 3D, so a good 2D workflow was more important to me than 3D features, and while Unreal is on a whole other level, Godot has been making big 3D improvements with the 4.x branch so it's definitely a viable choice for 3D as well these days.
All three are great options, and unless you know that there's a specific feature you're looking for (certain programming language, has to offer X out of the box, etc), it's probably worth just trying out each for a bit and see what you like. Maybe do a game jam with each or something?