It wasn’t clear to me from your spec if you are looking at one or two sliding doors. When I was looking for my van I had in my mind that I only wanted one. I ended up with two just because I found a cracking deal on a van and wouldn’t be without two now. If you have no plans to do a full camper conversion, I’d recommend two sliding doors. It just makes the van so much more versatile. I can also recommend the Ovano system. As well as providing a bed, it makes the cargo space far more usable.
As others have said, better to retrofit a leisure battery, you will need one if you’re going to use a fridge in there. Top loading portable fridge is a great option (there’s a whole thread on these to keep you busy).
https://www.t6forum.com/threads/fridges-cold-beer-how-i-done-it.20628/
Rear camera is a must (I retrofitted one), don’t bother with the automatic parking.
If you can afford it, LED headlights.
I don’t know if this is included in the Highline on the 6.1 or not (I have a T6) but Apple CarPlay or the Android equivalent on the head unit. I use this so I can use google maps as my SatNav with everything displayed on the head unit. I wouldn’t be without it.
Carpet mats and seat covers are also useful. I got these thrown in for free by the dealer.