Some observations
In general, using presets to go from is a good idea at any level of professionalism. But at the higher levels than level 1 you need to tweak them after finding the right one. At higher levels than level 4, you start to make your own presets. If you learn the basics of the compressor for instance, and learn what to listen for when tweaking - you can tweak them pretty darn fast. Which is a good thing in many ways. It takes some time up front, but it's not so bad if you see it as playtime
With regards to mastering: If you don't do much mixing, and don't have a semi-semi-professional engineer mix your stuff - I really wouldn't bother with mastering, and would definitely not be buying anything (neither DAWs, plugins or mastering services).
Also, each new DAW will have a learning curve. I have tried the studio one demo some time ago and liked it (also coming from reaper), but there were some things that took some time to get used to. In the end I went with logic pro x. Which also has a learning curve. If you don't get into it much beyond a certain point, I really don't see the point to switch DAWs. If you DO want to go with studio one, consider my above point about mastering - maybe you don't need to buy the professional version?
With regard to multiband compressor: This is a dangerous tool, that takes some dedicated time to learn to use well (generalising here). To me it sounds like that isn't what you're after? Is there a reason you want this specific plugin?
Anyway, de-essers and multiband compressors I am pretty sure you can find for free. Especially for reaper, which accepts a lot of different plugin formats.
Come to think of it, I think the ReaFir plugin can be used both as a multiband comp and a de-esser.
If you want better results - or just want to enjoy the process without having it feel like a lot of work - I would advise practicing with the tools you already have. With practice, it will feel less like work, because you free up your mental capacity to focus on the music, not the technical side of things.Also, don't mix while you record. keep the processes separate, if you don't already. Or at least try it.
I always give the boring advice. I suck at being an enabler.... :-/