Best upgrades for me was the following in order of overall impact

Klipper
BTT Skr mini e3v3 board
Silicon bed spacers
Klackender mod by KevinAkaSam
G10 build plate
Orbiter extruder and dragonfly hotend for direct drive
Belted Z mod by KevinAkaSam

More recent EBB36 canbus for the tool head mainly because it frees up some ports on the control board for other things like my Nevermore.

For overall quality and reliability the following have the biggest impact.

Ensuring the physical frame is as square as possible. As well as adjusting and shimming things such as folded aluminum foil under the Z extrusions to get them square with the base.

The board and Klipper are huge since it makes it easier to use the klack probe (Klicky for ender) and the silicone spacers allow me to dial in the screws with adjust_screws.

And skew correction because my X gantry is twisted (tested that on granite counter top) and after I’ve done everything else to ensure it’s physically right i still had to work around some of it with software.

Now I can just fire it up and print PLA, ABS (it’s in a grow tent enclosure), PETG and TPU without any issue.

The g10 plate works great with all of them, Klipper can compensate for any warping, build offset as long as the screws are properly adjusted (which I test every couple of months or after I’ve had a print that really didn’t want to come off.

I’m building a trident myself right now and I’m using the “Frankender” to do it with really great quality results.