Open source, both hard and software

It looks like Solvol checks this box.

You'll be able to reflash the vast majority of budget printers with more open software if the stock option doesn't make you happy. You can also replace the main board and choose your own adventure.

Does this mean that you're open to brands other than Solvol? If yes, one of the most open source printers out there are Vorons. They're a community maintained completely open source design that uses a mix of parts you print (or buy onnthe market) and fairly widely available hardware. They run klipper, so there's zero printer specific software. They're not that cheap to build though and they don't sell parts/kits directly. There are a ton of third parties that sell kits for their various designs. For example, the cheapest/smallestbprinter, the V0, is available for $320-$400 from formbot depending on the hot end and warehouse you choose.

Note that I'm not trying to disuade you from Solvol.

Part availability and software interoperability/ecosystem

Most printers are ultimately fairly simple mechanically and will be repairable, but doing so might require modding and/or effort by their owner. For example, you can retrofit the Voron extruder onto the Sv06 should the stock unit fail catastrophically and no longer be available.

You sound like a tinkerer, so you'll be able to fix your printer. I wouldn't worry a ton about this. Most popular models tend to have decent support in the marketplace, but when it comes to long term wear out the marketplace does shrink because the once-popular model will have lost its popularity.

Stability/quality of the build

People seem to like the Sv06. I've not personally used one, so I can't comment.

Print quality and speed

I've not personally used a Sv06, so I can't comment on print quality either. I can comment on speed coming from a different i3 clone, which is basically what the Sv06 is, to a Voron which is a pretty fast printer.

If you want fast extruder speed, klipper will help immensely with things like pressure advanced and input shaping. An i3 style printer will only be able to move, and especially accelerate, so fast though.

But honestly if you're looking to reduce print speed drastically larger print movement speed isn't the thing to chase. You want a bigger nozel and a hot end that can melt plastic fast enough to keep up. Larger extrusion widths mean you won't need as many perimeters and thicker layers mean fewer layers. Cutting the number of perimeters in half is effectively better than a 2x increase in print speed because shape geometry will mean you won't run at that top speed all the time.