Some variations are expected, and it’s hard to pin down why exactly. It could be the filament, it could be the linear module, it could be something external like if your printer is near a heat source and that’s how often the AC/Heat turns on. One of the best ways to start figuring that out is to redo the print a couple of times. If the spots appear in that same place everytime, it’s probably the machine. If they appear in different places but with the same frequency, it’s probably something external like heat or the filament. If it’s completely random, it’s probably the filament.
Nobody really trusts the Snapmaker PLA anymore. There’s been too many complaints of wet filament. You can try to dry it if you want (search the forums for “drying filament”, or try google/youtube). The recommendations are usually to not bother, and buy a better branch (search the forums for “recommended filament”).
I haven’t done PETG, but it does seem to be possible, just tricky. Search the forums for “petg”. There’s a lot of mixed results, and lots of good advice in the comments. It’s probably going to make for a lot of reading.
Lastly, if you haven’t seen it, xchrisd’s 3D Print Guide is a great place to start. It’s got levels of availability, so do the parts you’re comfortable with, and read it again when you feel like you’re ready for it. There are parts that are only for the v2 printer, but plenty of it applies to the v1 as well.