First up: If you were a machinist and can model in CAD (especially constraint based, not mesh based), slicers are trivial. You'll spend a bit learning about materials & how to deal with their inadequacies by fiddling with support structures, but it's really not an obstacle.

If OSS firmware and repairability matter to you, prepare to invest a lot of time fixing and improving your tooling. (But... if you've machined before, you likely know that drill :)

The Sovol SV06 is almost there in terms of build volume - and it clocks in at $350, so it might be a good starting point. The Voron 2.4 is 350 mm^3 and almost $1000, also a decent machine. Elegoo's Neptune 4 straddles the edge of open source - it's a branded klipper build.

All of them are enclosable (i.e. you can buy enclosures, or build one)

From what I hear, the Voron's the most likely fit for your material requirements. (But, worth keeping in mind - an alternate path is a cheapo PLA printer for prototyping, and then shipping of files to a printing service. Depends on what you plan to do)