Well, I've seen those brazed gold packages on Digikey... GeneSiC and such. Looks like they drop off around 225C, not 250C. I don't think anyone makes anything SiC under 600V, because... why bother, I guess? I expect it can be done (what's Vebo on a typical SiC BJT?), it's just not.
As for what you *would* use at 250C and lower voltages... does... Raytheon still make subminiature vacuum tubes? :o
I expect you're asking for something so rarefied that manufacturers don't bother talking about it. Useless to ask here (unless someone here has already done it -- AND isn't under an NDA!), or even at suppliers, likely...
I'm not familiar with the vast world of downhole (and related) electronics, but it's true in other market sectors that, you're probably stuck with asking mfg reps and quoting and sampling and NDAing and all that bothersome old timey stuff.