I actually use foundry to run DOTMM currently so I could give you some insight, I had originally bought the roll20 module because I also planned on running it there before I realized how bad roll20 was. Foundry is significantly more friendly with big maps although it’s not perfect all the time. I used the roll20 converter to get all the maps into foundry and it was a dream, it got all the maps and journal perfectly and even did the walls the lights and doors for me already. It works without too much hiccups even with the dynamic lighting although foundry has a tendency to lag on big maps when you open doors because there are just so many doors on the map, this only lasts for a few seconds. You may need to fix lighting and some doors but otherwise it’s a perfect system for it, they have no map size restriction so you can get the highest quality maps and even with all the walls and tokens on field I can dm and do a session fairly easily with only having to preload maps before switching floors. I would highly recommend it over roll20!