Decided against the R5 in the end, the R6 II looked really good but the overall costs are still pretty high and the battery life not up to where I want yet with mirrorless, and the rolling shutter still is a thing unless you go R3 which is twice the price basically.
I ended up buying the 5D4 at £1500 and also ordered a 100mm 2.8L Macro IS as just sold the Sigma 85mm 1.4 EX.
5D4 has usable ISO 12,800 so this is fantastic as I shoot un-flashed a lot in low light so knowing I have that much ISO range now without worrying about noise is excellent. Improvements over the 5D3 noticed so far is that better lens corrections built in, AF system is obviously better, more customisation and more details on through the viewfinder.
The macro lens has been a long time coming, been using the 35mm 1.4 L II for close-up stuff and it's been fine, but it's not the magnification I really want to play with, so the lesser used 85mm 1.4 had to be shifted on to make way for the 100.
Think I will be fine until the next gen of mirrorless come out DSLR battery life and stuff.
Slightly annoyed that the 5D4 only comes with an E6N battery not an E6NH as I bought a H ages ago so was expecting to now have two lol.
So ended up with a 5D4 + 100mm 2.8 L IS for less than teh cost of one R5 or R6II body only - That'll do me.
That 135 F2L on the 5D3, that lens has been absolutely amazing for such an old lens. no weather sealing or anything, just pure optics. Sharp edge to edge and so fast to focus.