I probably won't be going for commercial use, just hobby stuff mainly. It would be neat to do some nylon stuff but that's probably as crazy as I'd get.
But yes, ideally my budget is $450-1200 (wide I know, but I'd like to explore my options).
I don't think the MK4 is a good value in this case. You are already familiar with setting up and maintaining a budget printer as such it just works of Prusa isn't as critical.
The answer might be your current printer: Octoprint and opensoucre are possible, bed leveling is build in, enclosure can be build but not ideal for bed slinger and size upgrade is the only point not feasible.
What are you looking for if this isn't an option?
Enclosure -> coreXY, etc. | no bed slinger
OpenSource -> no BambuLab P1S
OctoPrint -> this is rough. Most printers ship with Klipper. Maybe a DIY-kit so you could choose.
larger than neptune 2 -> larger than 220x220x250mm
pre-built options?
$400: Kinggroon KLP1: smaller (220x220mm), no Octoprint
$600 FLASHFORGE Adventurer 5M Pro: no OctoPrint (klipper), identical printvolume and only klipper part of the firmware is opensource
$600 creality K1: no OctoPrint, identical printvolume
$700 Qidi Tech X-Plus 3: no Octoprint
$900 Creality K1 max: no OctoPrint
if you are lucky and max out the budget a used E3D tool changer could be possible (don't know if Duet 2 [reprap firmware] is compatible with OctoPrint)