How to determine fire hazard of heating ethylene glycol to dissolve borax and boric acid
Chlorates, perchlorates, permanganates and other strong oxidants conflagrate even at room temperature with ethylene glycol.

Boric acid and borax, however, are not considered strong oxidants and should probably be safe to dissolve below the boiling point of water. That said, I would test a small batch in a safe place, to be sure there is not a large gain in temperature on dissolving.

BTW, you might be interested in reading of the preservation of the Swedish warship Vasa in polyethylene glycol, and long-term issues with acidification. N.B. Polyethylene glycol is generally considered innocuous, but ethylene glycol is toxic if ingested.