It's always more than you think I have found

And better a metre too long than 10cm to short

At the lengths you have quoted below, I would be happy with 16mm2 cable. In actual fact, the next standard step up in size is 25mm2 and the Orion doesn't even accept a 25mm2 cable directly, so
Victron don't really expect a bigger cable to be used normally!
What I think is missing from the above is a note of what Neg/Ground you have between the Leisure
Battery and Starter
Battery?
If it is not upto scratch already, or you want to make sure it is good enough, I would have a 4th cable - from Orion B2B to Leisure
Battery. So what I would suggest is the following:
+VE - Starter to Orion IN - 2 Metres
+VE - Leisure to Orion OUT - 1 Metre
-VE - Starter to Leisure - ? Maybe 2.5 Metres? Maybe 2 Metres?
-VE - Leisure to Orion COM - 1Metre
(this way you would only have a single ground connected to Orion. Two cables into one Orion terminal would be awkward to manage and service)
You need a
fuse at the Starter
Battery +ve end and you also need a
fuse at the Leisure
Battery +ve end.
I would also recommend a
fuse at the other end of that 2M cable from Starter to Orion IN. Strictly speaking it is not needed but I tend to put a
fuse at both ends of that long run as a precaution.
What is the cable run like in that metre between the Orion OUT and Leisure +VE? You might want to consider a
fuse at the Orion OUT possibly if there is any risk of cable damage in that run?
The above would mean a total of 4 fuses so bit more than typical but makes the install very safe.
Ref the terminals, looks like a mix of 5mm and 6mm bolts there with possibly an 8mm on the last one? If you can confirm the sizes needed at each
battery ends, that would be good. Not the biggest fan of stacking lots of cables on a
battery terminal, but if supported well, it is ok.
The +ve leads at the
battery ends will have a short lead from
Battery +ve to
Fuse, so if you are switching in the future to Lithium, that will almost certainly need 8mm rings - I could send a couple of loose 8mm lugs to swap over when the time comes. You could potentially have 8mm to start with if you can arrange the connections so they are securely clamped down and making a good connection.
I'll check what cable I have in stock - I think I have what is needed and that would save extra shipping costs to me on a new order and so save you a bit of money as well.