I guess figure out the average amount tumbled typically (maybe half a bitcoin - 4kish), divide that by total amount (4.5m), something like 1100 tumbles/mixes? Then spend 1100 days mixing the total sum to keep things as banal as possible (3 years). Possibly randomize the amount tumbled per transaction up to $1000 dollars.

The 4.5m would have to be mixed immediately so the original wallet can no longer be an event source.

Is that the right idea?

The smarter move if this was 1 person who intends to use the 4+m to live off, and not buy a Lamborghini or some stupid shit, would be to extend the timeline much further past 1100 days. The smaller the individual amounts the better. If they are long term bullish on the prospects of bitcoin retaining its value or increasing, launder just enough to pay themselves a nice monthly 'salary' for living expenses, under 200k a year, and take a lot of time to consider what to do with the rest.

And dividing it up between as many possible independently run tumblers/mixers.

I'm sure some additional smurfing techniques could come into play for additional obfuscation.