I totally get the warm beer argument but don’t let this split charge vs DC-DC charge issue detract from your enjoyment of what is still a great van. The leisure battery should definitely last more than 2 years - ours, apart from the regular driven part, is a similar set up to yours and appears to be still going strong after 3 years (though we do have an issue with the solar panel no longer charging when the sun shines but I think that is the charge controller - trip to See
@travelvolts soon me thinks!!). Up until a couple of months ago we had the fridge running 24x7 for months on end without EHU and the van standing without being started for weeks.
Ultimately you will need to balance the cost of replacement leisure batteries (circa £80 for a 75ah AGM that fit under the seat with the Eberspacher outlet) with the cost of upgrading to a Dc-Dc charge system to get the extra 20% (£100 upwards).
Longer term we will be looking at lithium but not until the original AGM no longer does what we want.