To upgrade to 3.x/4 or to mod?  

To upgrade to 3.x/4 or to mod?

I've had my mk3 since 2018 & loved it - its printed through whole countries of filament, is a reliable old workhorse, and I bought every upgrade pack right up to and including the Mk3S+. But I baulked when I looked at the price of the upgrades for the newer upgrades because, well, seems a bit wild that the v4 upgrade price is only a couple of hundred euro off the price of a mk4 kit.

Up til now the most radical thing I've done with my printer is put a copperhead bimetal heat break on it instead of the "prusa specific" & changed the bearings to the newer ones Prusa started selling a year or two ago. I never bought the MMU kit because I built the entire thing from the original kit & it took a lot of time & effort to get it to the point where it prints perfectly every time. It fact it prints so well, that even the few minor issues I did have disappeared after I moved home to a far more airtight home & all of a sudden had no more issues with bed adhesion - so I was unwilling to make a change on the scale that would see me dismantling the printer too much.

In contrast I've 2 other printers, both Crealities, & I've literally changed everything on them from the boards to the extruders, and in one case the hot end (and even there I also switched to the Copperhead CE heatbreak).

So I am looking at my options now that the Mk3s+ is about to stop manufacturing. I've really liked having 100% support from Prusa (they even redesigned one of the hot end fans after I reported an issue with that version and offered to post the new version to me in Ireland). Having 2x printers from another supplier who vanishes away older versions like they never existed with no upgrade paths, I'm wondering if the mk4 upgrade is the best path.

Theoretically I could switch out the extruder, hotend, and even the board to newer versions that potentially match the mk3.5/mk3.9 improvements. There's now so many options that simply didn't exist when I bought my Mk3 back in 2018 - there's at least 4 really promising upgrades I can see just for extruder/hotend and at least 2 boards that would probably work with some customising firmware.

So what do you think? Has anybody who has upgraded to an e3d, Bondtech or Slice Engineering alternative regret doing so now that Prusa are offering their own upgrades? How do they compare, for anybody who has more than one Prusa with different upgrades?