It would be ridiculous to scrap the entirety of the voyager fleet at just 25 years.
Assuming that we're going to keep the Voyagers slightly longer term I'd be inclined to get (say) 12 nine coach 80x (more would preferable).
You could then scrap a load of end vehicles in the 221 fleet and create a fleet of 6 coach voyagers (26).
You could then scrap some end coaches from the 220 fleet to create 17 units with 6 coaches.
Those 6 coach trains could have 306 standard class seats and 26 first class seats (total of 332 whilst a pair of 4 coaches has more at 400, not that many extra). Even if they were to lock out of use the rear set due to a lack of staff, it should be tolerable.
That would give you a combined fleet of 43 Voyagers which could almost entirely be paired up. Although, there's scope to run 6 coach units towards the outer edges/on the much quieter services if a smaller unit was preferable.
Both the 9 coach 80x and the pairs of 22x's would have around 600 seats, so they could stand in for each other if you didn't allow all the seats to be bookable to balance the difference (802/1 had 647 seats, of which 71 are first class whilst the 22x's run in pairs would have 664 seats of which 52 are first class).
It's also worth noting that currently XC are leasing 312 coaches across 70 units, doing the above would mean that they are leasing 366 coaches across 55 units. As such the lease costs wouldn't be significantly higher but the trains would have a lot higher capacity.