So, debrief for my first #soldering job:

Things I did right:
- Safety: got the safety glasses, FPP2 mask, did everything in a well-ventilated room with the windows open and even had a tiny fan to top it off... still burned myself with the iron two times though.
- Tested the circuit on a breadboard first to know what to expect from the finished circuit; this allowed me to identify that one of the potentiometers was not correctly soldered (and I just identified there's another one that has a very weak electrical connection... but I'm not quite bothering to fix that for now, read on).
- Technique: I mean, not the best, but I think I got the basics of how you gotta do things. Kudos here to @Blenster for teaching a bunch of us on PancakesCon the principles of how solder works and what to look out for; I never quite understood the technique until I understood the principle.
- Good solder alloy selection. Sn100Ni+ is neat, seems to behave really nicely… BUT! (see next section)

What went wrong (basically everything!):
- FLUX. FLUX. I NEED FLUX BADLY. I thought I could get away without it for a simple through-hole challenge with a decent lead-free… NOPE, NO WAY, I REALLY NEED IT FOR LEAD-FREE, OOPS.
- Measure twice, cut once. Didn't even bother to read the instructions before starting in order come up with a game plan because I already read them; ended up putting the potentiometers backwards.
- Living with harmless mistakes: the circuit worked perfectly even with the aforementioned mistake, but I couldn't live with putting the potentiometers backwards; fixing it ended up being an agonizing 1-hour rework job (made worse by not having a solder sucker) that nearly turned the initial success into a complete failure, and overall just made things way worse.
- Sn100Ni+ is a neat lead-free solder alloy, seems to behave really well, BUT it seems to have a bit of trouble getting to the other side of through-holes when soldering, instead it tends to plug the other side then starts to pile up. Lots of through-holes left kinda half-done because it was really difficult to fix this for components like the potentiometers. Not sure how much of an issue this is.
- No-clean flux is horrible, "no-clean" truly means "uncleanable dark poop blobs" even if Felder markets it as "clear" flux. To be fair, this was the only flux core I could find for this solder alloy. For now I guess I'll live with it, but good news: the shop where I bought the solder now has more flux core variants for that alloy, so I'll keep that in mind next time.
- The tiny RGB LED through-holes were straight up Solder Bridging Central:tm:. Again, need flux badly; flux would've probably made everything flow nicely and I could've probably soldered them in a sweep… probably.

Next steps:
- GET FLUX.
- Get a solder sucker.
- (Maybe) get helping hands.
- (Maybe) get one of those cheap microscopes with screen; right now I'm pulling it off with my phone and it's not quite the best way to go, to say the least.
- Get a protoboard and a few sacrificial resistors, use them to study the solder alloy's behavior regarding through-holes and understand if and how adding extra flux can help with the issues I had.
- Do the SMD kit I have once I feel like I am ready for another go.

Overall: still got a LONG ways to go, but I definitely feel a bit more confident now, even despite the terrible and time-consuming mistakes.