IMO the best painting/lightweight editing tool is paint.net. Since I moved to mac I moved to Gimp and it’s ‘fine’.. but paint.net is really the only tool I’m missing fron windows
Spend a little bit of money on pixelmator, you’ll be glad you did. If you can afford a mac you’ll find that buying pixelmator to replace gimp is worth it.
There are a few really good low-ish cost options on Mac. You should take a look at Pixelmator, Acorn and Affinity Photo. I think all have free trials. Out of those, Affinity is the one I use most, but that's because I also really like Affinity Designer (an Illustrator type application).