The wisest choice.
I myself have three Gravemakers, the first obtained way back in 2019 when he was one of the featured heroes in an Atlantis portal. Got my 2nd GM from the first ToL event, and the third GM from the second ToL event. I maxed the 2 GMs I have, one emblemed to +19, which sometimes appear in my regular raid and war defense as I change and swap heroes in defense on a regular basis just for trying them out. Apart from farming and emblems quests, The 2 GMs I have serve in my second monored team flanking my unemblemed Ares for his attack and critical buffs and used mainly on wars against most green tanks. Yes, their burn don’t stack, but I try to distribute their specials on different sides after killing the enemy tank, damaging the enemy heroes on either side while dealing some more damage due to the burn in 2 turns. That’s the best thing I can do with 2 GMs running in the same offensive team. I know some players have at least 3 GMs maxed and I hesitate maxing my third GM even if I have 28 rings in my inventory as I see them reserved for either Marjana (I have her at 3/70) and her costume, Vanda, Garnet, Black Knight, Gefjon, and even perhaps Tyr. If my rings get to 36, I may ascend my 2nd Mitsuko also left at 3/70. Like you, I dont have many great choices in my bench, i.e. dupe Azlars, dupe Khagans, dupe Reubens, and a 3rd Mitsuko.
Good luck and may the RNG gods be with you.