Murderous Rider > hero downfall

Strionic Resonator to copy commander ETB, I see you already have Panharmonicron so it might not be necessary but redundancy could be useful

Aetherflux Reservoir to combo with top and bolas

What has been your experience with doubling cube? It seems win-more but then again, you do make a lot of mana. I see you also have Erebos and Arguel blood fest. Do you need the redundancy? I would say Necropotence fits better than blood fest

Blast Zone to get rid of problematic artifacts and enchantments if you can’t get meteor golem out

If you want to be janky, Kormus Bell urborg and massacre Wurm are a fun time XD

Infernal Darkness to cast on mainphase 1, then go to combat and mainphase 2. The opponents will need to spend mana they tap in response to infernal during MP1 or else it will fall out of their mana pool as you switch phases. This helps prevent interaction during your turn (aside from artifacts and mana dorks)

Command Beacon to help reduce commander tax

Overall I like this deck. I would personally go with 1, maybe 2 back up combos if all else fails. Any more might subtract from your overall game plan. +1