I think the best way to handle this kind of content is to make it 5-8 man content.

I understand that you want strain as a healer, but that's not what is being said. It's not that difficult 4 man content puts too much strain on a healer, its that the stress of playing a healer in this kind of content is massively disproportionate to that of tank or DPS, meaning that the content would always consist of long queues of people waiting for a healer, as everyone will queue as the easier roles.

The 8 man system works, and I would love an 8 man dungeon (like castrum/praetorium) as our difficult dungeon content.