Questions:
1. Is the case fan on the SD3 required? I bought a replacement, but if I can eliminate a fan I wouldn't mind.
2. Has anyone tried a pancake Nema17 for the extruder motor? I'm wondering if I can gain some accuracy by reducing the carriage weight. The pancake nemas weigh about 200g less than the standard ones. The specs seem very similar.
3. What are your thoughts on upgrading the PCB? I'm interested in upgrading that as well if it will lead to better p
prints, but the reviews seem mixed on if that will help or not.

1) I see no reason it can not be eliminated . IIRC the sd3 was not even enclosed like the sd4 is.
2) The motor should give some improvement make sure you get a 1.8 degree motor not a 0.9 .
also a titan or aero titan combo can give some improvement.
3) As always your dime your call. IMHO if you are going to upgrade go for a 32 bit board that has similar connectors and similar wiring  to what you are using. A 32 bit board simply handles things better. the arduiono board is great but a bit long in the tooth. Take a look at teaching tech videos . this is the same mike lawsey that uploaded the carriage design years ago. He has lots of videos on upgrading 3d printers. The reason I mention the similar connectors and wiring it will save time and hassle making the swap. That way youdo not have to reconfigure your connectors.

4) there are many options for print surface gecko tech has a good rep. Ultimately you need to go with what works for you.

1.Lawsy carriage + rods round 1 (I used to spray aqua net on the glass bed while it was in the printer, I know now this was not the best idea... I've learned a lot since those dark days)
2.E3d hotend
3.Updated bed heater (the one from Printit)
4.Updated PSU
5.Been using LokBuild print surfaces since kickstarting it years go (love it)
6. Filament dryer

I'm looking to do some updates to my SD3. After years of printing with only ABS and having issues, then years of letting the printer sit I'm back to printing Switching to PLA and PETG has been super rewarding!

Upgrade list:
Re-printed Lawsy carriages
Replace rods and barrings
buildtak flex plate bed
M5 z-rod replacement
part cooler fan

Questions:
1. Is the case fan on the SD3 required? I bought a replacement, but if I can eliminate a fan I wouldn't mind.
2. Has anyone tried a pancake Nema17 for the extruder motor? I'm wondering if I can gain some accuracy by reducing the carriage weight. The pancake nemas weigh about 200g less than the standard ones. The specs seem very similar.
3. What are your thoughts on upgrading the PCB? I'm interested in upgrading that as well if it will lead to better prints, but the reviews seem mixed on if that will help or not.

Thanks as always for the support