Nice trenching! Are you dropping in just copper because its less than 100m? Or are you putting in fibre as well?
>Ok one question. Why is 10GbE copper over RJ45 (And it’s later variants) so much more expensive than SFP+ kit?
I haven't seen anyone use Cat7.
Cat6a is normally suitable for 10Gbe copper, and it should not be that expensive. It is normally cheaper than fibre and 10Gbe SFP+ adaptors ... but in your case you would probably need to use externally rated copper in a special cladding (sometimes grease filled), and yep that costs more.
If you use fibre - you can either put it in conduit or buy it with an external cladding and then yoiu can drop it straight in. You also don't require spacial seperation for fibre (although your might have local building codes or regulatory requirements for this).
With fibre you can start with Gigabit SFP adaptors, and then at some point in the future upgrade for 10Gbe SFP+.
You might want to think if it is worth going to 10Gbe straight away, or weather you are doing 10Gbe because you can and it would be a "nice to have". After all, everything is currently running over a a WiFi link ...
I'd be tempted to drop in a 6 core fibre. You could initially use a pair of them in a LACP channel. You could use cheaper MS120-8(P)'s at each end with Gigabit SFPs. The MS120-8's also come in a PoE option which could be usefull in your case. Of course, you might already have an MS at each end.
If you jump all the way up to an MS225-24(P) which has 4 x 10 SFP+ ports. You use 10Gbe fibre between them, and then plug in your NAS via 10Gbe SFP+ to the MS225.