there is no point making a glue joint that is stronger than the parent material. It's a common error.
Normal PVA woodglue makes a joint about as strong as timber, but it relies on the faces being hard against each other, so you need to make your joints flat and square. If you have substantial gaps in the joint then your problem is filling not glueing. Epoxy with a filler is about as good as anything for that. Araldite, or epoxy mixed with wood dust or filler beads are common options.
Finally PVA into endgrain can get soaked up leaving inadequate glue in the joint. Best to use extra on endgrain.
If you really want to make your corners stronger some triangular blocks into the corners makes the glueing area much larger and the whole thing stiffer and stronger. Clamps to hold them in place, spring clamps are cheap and easy.
I'm just a startled bunny in the headlights of life. L.J. Young.
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