I use Dungeonscrawl for a lit of my prototyping. It’s incredibly fast to use, available everywhere and mostly free to use.

However, if you ever consider making serious maps, that look at least halfway decent then there’s really no way past payed tools.

I personally recommend Dungeondraft. You pay $19.99 once and can use it forever and without any limitations. It runs flawlessly on linux and there’s tons of free assets out there that work out of the box. Honestly, the second best spent 20 bucks of my GMing career. Number one is FoundryVTT.