An easy way to explain RDJ being Doctor Doom is if it’s established that some variants are actually doppelgängers of established characters. Like as part of the chaos of the multiverse, some people end up looking like a completely different character in another universe. To retroactively justify this:
1) MCU Kate Bishop’s doppelgänger is Gwen Stacy from Spider-Verse
2) Apocalypse’s doppelgänger is Marc Spector/Steven Grant/Jake Lockley
3) Deadpool’s doppelgänger is Hannibal King from Blade Trinity
With this explanation, we also justify why certain characters are played by different actors (ex: the 3 Spider-Mans).