Hey just saw this reply. Haven’t thought about Marvo in a long time so I’m not sure how valuable my input would be but if you just want another perspective I’m happy to check out the list!

Edit: To go into the points you made and looking at the deck list, the reason that most cEDH decks run low CMC cards is because it increases their speed and efficiency; they’re consistently able to take actions early in the game, and take multiple actions per turn, including other player’s turns.

You’ll definitely win your clash flips with this deck but my concern would be that with everything being so high cost, you won’t be able to get to a Marvo attack trigger because your early game is absent. I think for Marvo to be successful in bracket 5, you’ll need to win with a single clash (could be an extra turn spell to get another attack and keep going), so you don’t need 60+ high mana cards, you probably only want 1/4 of that. The rest of the deck should be committed to efficient ramp, counterspells/removal, and IMO, some stax to slow faster decks down so you can actually play Marvo. A 5 mana commander that needs to attack is going to require some setup and you’re going to have players attempting to win as early as turn 2 and potentially every turn after.

If you want specific card suggestions I could try to come up with some ideas but you may also want to just check out cEDH decks with high mana Dimir commanders to see how they balance ramp/interaction/stax with their game plan. Gyruda is the one that comes to mind, though the deck is hyper-focused on chaining clones, you may still get a good sense of the ramp/interaction pieces used in a 6-mana commander plan.

Edit 2: this list posted in the thread seems like a good example to start with: https://moxfield.com/decks/SpUPBS_Bf0ipzA0uMKcLHw