Cotronics makes castable ceramics. Start there.
It took a year of University level R&D to make a low firing frit that did not use dangerous amounts of nanopowder lead oxide. [ The stuff floats in air currents.]
Boron based three part glasses melt at 890'c in theory. In reality they are very, very, very, brittle. When you add even one percent of another oxide to make them useful the melt point goes through the roof. Yes, you can make a three part glass with. hardware store and drug store materials. Is it workable, no. Does it pour well, no. Does it stick to the crucible ; oh yes.
Making one that did not use Antimoney and Arsinic compounds that vaporize to encourage melting is difficult. I ain't talking.
Solderable low temp inks are proprietary. The artistic firable gold, silver; and copper inks for kiln firing contain bismuth as a catalyst and will dissolve before they solder.
Good adhesion of conductive inks in "cheap" processes usually requires a layer of nickle under the metalization for adhesion.
Home made "dental cements" that are water based are a thing of the past. Work great until you need to fuse them.
By the time the R&D was done, my supervisor agreed to have a second US Citizen Permanent Staff member added to the team so we could order sample quantities of LTCC materials, which are highly restricted in the US due to treaty. The two US Citizens would have had to keep the material in a safe and log what they dole out to the non-citizen students and professors.