What an awesome project - thanks for sharing the details!!

Two things I’d suggest:

Try another brand of PETG - I’ve heard weird things about MatterHackers PETG but never used it personally. Overture, Polymaker, Prusament, and Printed Solid PETG always behave consistently for me.

Take a look at your heatblock and make sure you’re not leaking anywhere. I tried a 0.8 nozzle when I first got my MK3S+ and got bad results b/c I was leaking out the top and it was stringing badly. I eventually went with a Revo nozzle because swapping is so much simpler.

After looking at the upgrade options (and shopping with your wallet, not mine), I’d say buy a MK4 kit. You’re in a fairly unique position that you aren’t taking apart a working printer; you’re getting a new printer with 2023 electronics and motors for a discount.

However, if sinking that kind of cash isn’t wise for you right now, then build what you have! Or, pick up the MK3S+ upgrade kit for $50USD. MK3S+ is a reliable workhorse, and mine won’t be retired anytime soon. It still prints PETG as well or better than my X1C, if a little slower.

Good luck and have fun with it!