Its what I'm thinking too.The Rotel has no shortcomings in terms of sound at its price, and that Burr Brown DAC sounds superb. If Rotel could get the reliability up to scratch there wouldn't be any question of need a replacement unless your budget got significantly bigger. Then a demo would be a good idea!
As it goes, I'd suggest plumping for the Marantz CD6007 and set to enjoying your music. That's where my own thinking lies at the moment - Dad has the 6007 pairing (with B&W 607s2) and it sounds superb.
What bothers me about the Marantz is the rave reviews are all based on the original AK4490 DAC, which is no longer used due to the supplier having a fire. Marantz have since substituted it for an ESS DAC since about 2021. It doesn't look like anyone has reviewed or tested the new version, but they surely won't sound the same.
I notice a few people are complaining the treble performance is harsh which makes me wonder if it's related to that DAC change and it's not the crowdpleaser it once was.
Did your dad buy his 6007 before or after 2021 or do you happen to know which DAC is in it? If it's the new one I'll probably grab one myself. If it's the old one I might pause and reflect for a while or consider some of the other options you mentioned.