Second hand market for prusa printers is decent as well.
The Mk3.9 probably isn't a price competitive upgrade though, today I would in your position probably look at selling the Mk3s for 100-200 EUR and buying a Core One instead. But if the Mk3s works for what you are doing, then don't bother. (The Mk3.5 might make some sense if you only want a bit higher speed and network connectivity.)
Personally I don't see myself replacing the Mk3.9s for a long time, though an enclosed printer would be nice and let me print warp-prone materials like PC and ASA.
Because of things like acceleration and flow limits the scaling isn't as simple as a constant factor but also depends on the geometry of the part (sharp corners lead to slowing down) and material (check max volumetric flow in your slicer, will likely be set way too low for your current filament profiles, especially if you switch to a high flow nozzle on the nextruder).