I personally do not since I use plugins like SoundTheory’s Gullfoss and Oeksound’s Soothe2, both of which Reaper simply doesn’t have 1:1 alternatives for, but I’m sure some here do…
For me it’s all about problem-solving. If Reaper’s default plugins solve the problem, I’ll use them first since they are fast and efficient; however, there are plenty of problems that would take far too long to solve with Reaper’s plugins and automation (such as how Soothe2 solves complex dynamic resonance issues)
Ideally though, mastering should take minimal work if the mix is right, but a ton of people tend to give you kinda-crappy mixes and then expect you to polish the proverbial turd (ok maybe not THAT bad but you get what I mean). If you are doing the mix though, then mastering shouldn’t be much more than maybe tweaking EQ a bit, MAYBE some light compression and limiting to get to your required LUFS, but that’s it. Reaper’s plugins can easily handle those three things