I'm a rookie setting myself up for failure. I've only built a HG grandpa, trimmed the nubs, sanded down marks, and panel lined with gundam markers (the non-flow ones).

That's more than most people do on their first kit. Or, if you're like my nephew, the 20th kit. I took him to a FLHS for his birthday, and his eyes lit up when he saw the ZZ ver. Ka. Put it together in one sitting and he's pleased as punch. Internally I'm screaming that it needs panel lines etc, but it ain't my kit lol.

Feel like I need some sort of pathway planned to build up the skills I'd like to have first.

Just build, I'm serious. If you have something you want to do up "perfect," save it for when you're confident in your skills. But in the meantime, do other kits. Hone the techniques you know. Push boundaries where it makes sense. For example, let's say you have an HG with minor color correction needed, like some thrusters molded onto a panel. You can leave it as is, use the included stickers, or paint. You decide to paint. Gundam marker? Hand brush? Mask and spray? You've got options. And each time, you're getting more comfortable, and building your toolbox (both skills and literally). Each kit is an opportunity to grow.

20 some-odd years ago, I dealt with nub marks by dabbing a little paint over the stress mark. Yes, it looked awful lol. Tried sandpaper but was never happy with the finish afterwards. I learned later that there were much much higher grits. Over time I got fairly decent at shaving nubs down with a hobby knife, then discovered single bladed nippers. Even with godhands, I still need a straight blade or some sanding to get a nub perfect... but most of the time I just go with "good enough."

Don't overthink it! Just enjoy the process.