The first step would be to test for adhesion. My assumption is you are not having high adhesion with the VHB Tape you’re using or you do not have enough tape to withstand the stress load. 60C should not be an issue for VHB provided you have high adhesion (Onyx is in question, we have good bonding to Nylon materials) and are using enough tape (i.e., following our design guidelines).
If searching for a 2 part epoxy, there are several options available but a good place to start would be to try 3M™ Scotch-Weld™ Epoxy Adhesive DP420 Black. Also please look at a paper topic on this: Joining & Bonding of Composite Parts –
The Structural Adhesive Advantage.