The general principles of choosing cables and fuses are:
1) Choose a cable size which is comfortably rated for the maximum current the cable will realistically need to carry. (With longer cable runs, the voltage drop might also come into play as a consideration).
2) Choose a fuse which is comfortably under the maximum rated current capacity of the cable (such that it blows first, rather than the cable) but over the amount of current the cable will realistically need to carry in normal operation. Situate this fuse as close to the source of power for the cable as possible.
Quite often, for specific devices, both cable size and fusing will be specified in the manual.
In terms of some of your specifics:
Isolator - Yes you can fit one of these if you think it would be useful to isolate the whole thing. The downside is additional expense, additional space required and an additional component to go wrong. I don't have one and after eight years using the van, have never missed it.
Main fuse - Yes, as
@EAN says above, fitting an MRBF or T fuse is a good idea, as close to the battery terminal as possible. It should be sized following principles 1&2 above. Note that this isn't equivalent to the sum of all downstream fuses both because it's not realistic for them all to be pulling the max current at the same time, and because you have loads and chargers which will net flow off against each other.
Distribution board - Given you're going to use a main fuse as above, your input fuse on your main distribution box isn't really doing much. Given that you could potentially use something like this (
Module For 6x Maxi Blade Fuses) which would give you six outputs and enable you to add your amplifier circuit (although see point below) directly on to there with a spare output for expansion later. Note - I haven't seen a max current capability for that unit I linked to, you'd need to check that, but given a single maxi fuse itself can go to 100A, I wouldn't imagine it's below that.
Amplifiers - Are these really a 80A draw?? If they're drawing anything like that in normal operation, I wouldn't put them on the leisure battery circuit. The reason is that if you're using these solidly whilst driving your DC-DC charger could be spending a big chunk of its 50A just keeping the amplifiers going, your leisure battery charging will be heavily impacted and you might even be draining your leisure battery whilst driving (depending on just how thumping you like your music!). If the draw really is going to be anything like that heavy, I'd put them on your starter such that the alternator is effectively powering them.
Secondary fuse box - This will happily provide fuses for all your smaller devices - just follow the principles 1&2 above to choose fuse and cable sizes there. As
@EAN says, make sure the fuse that feeds this in the distribution box is under the rating of the secondary fuse box itself. Don't skimp on the cable size for the fridge, they can be sensitive to voltage drop which can come into play on longer cable runs.
Units - You've used A and Ah somewhat interchangeably above. The amps (A) is a measure of instantaneous current whereas Ah is a measure of energy (assuming a nominal voltage to transfer to Wh anyway) - it's the amps (A) you care about for the purposes of cabling and fusing.
Other fuses - just check them individually, there are still some odd ones, for example, I'd expect the 50A one on the input side of the DC-DC to blow in normal operation assuming that's a 50A DC-DC.
