Cables from panel size

Solar panel strings are usually on 4mm² (2 per string)

I have a 5 core 6mm² SWA going from the barn to the shed where my panels are (about 50m away)

100% agree with Graememk as with pulling 2 sets of 4mm solar cable along with am Ethernet cable to my install outdoor it wasn't the easiest and that was with a 50mm ducting. Last thing you want is kinking and damage to cables or worse again getting to that stage and realising you can't get the cable through

Sounds like you're doing it all yourself but just in case you're not and looking for the grant. The PV cable and power to isolator switch have to be in seperate conduits - at least that's what I was told! It may or may not be useful to you.

What’s the max distance you’d consider for 4mm dc cables for say 4-6 panels…50 or 100m be too much? Looking at possibility of small ground mount but would prefer keep inverter indoors

Would you not consider micro inverters for the panels instead of a full size one indoors?

I'm considering doing. Ground system and using them.