Can I mod games as freely and as easily as I do on Windows?

It depends a lot on the game, but in my experience not always. Running games straight from steam works really well with a small number of exceptions, but a lot of the sometimes weird tools for patching exe:s and so on that some games use can sometimes be a pain to get running. Not necessarily impossible but yeah this is a reason for why I still keep around my windows installation for dual booting.

If you want to test windows programs on linux, you’re probably going to want to do that in a virtual machine, or even a spare computer just for testing on windows. Depending on how much you need to use excel, a virtual machine could be a good option for that as well, but if using Microsoft Excel™ is a big part of your job, maybe it makes more sense to just stay on Windows for work at least