You mention cost is an issue, in which case why do you need wifi7 outdoors - just settle for a cheap WiFi 5 or 6 AP, 6ghz range isn't great and do you really need maximum speed outside in your garden?
You just need external grade Cat6 utp.
Hey - I didn't quite say cost is an issue, just that it adds up. You know, you look at all the little bits you need to make the perfect system and suddenly you're a grand in.

On cheap APs, are you suggesting cheap Ubiquiti ones or other brands? What attracted me to Ubiquiti most was the understanding (perhaps incorrect) that the way the handoff between APs was handled centrally by the gateway was impressive and meant devices are always connected to the best AP (not what my heath robinson Asus setup does - it stays connected to whatever AP until it loses that signal completely and then reconnects to what it can).
So question is, do I need Ubiquiti APs or could I use my Asus router in AP mode on behalf of the UDR7 and get the best treatment?
If it's Ubiquiti I need, I think you just end up in the buy nice or buy twice argument. I don't need wifi7 outdoors as such (noting that I don't believe any outdoor-rated APs have 6Ghz) but could have an internal AP inside the outbuilding - speed more relevant as I have a TV down there which can be used for streaming from a media server in the house. There's a switch fed by a CAT5e cable however.
Whilst also brings me onto the CAT6 vs CAT6a point. I have an opportunity to pull the floors up whilst we're having new carpet fitted so wanted to get a decent bit of cable running around. Fully appreciate that I don't need CAT6a but since this is a one-and-done situation, whether there's any harm in a higher-spec cable. You never know what might be needed in the future, e.g. perhaps CAT6a will support 100gbe over shorter runs in 10 years time and it will be relevant!? So questions are more whether a) there's a downside and b) grounding is hard to do (using something like a Trendnet TC-P08C6AS which I can earth to the screw on the electrical faceplate). If it's a nonsense idea, CAT6 is fine as you say (F/UTP CAT6 is what I'm mostly seeing, which again I'm assuming doesn't need grounding)?
For completeness, the drop to the outbuilding is probably ~60m through plastic conduit, partly underground and partly overground. I might want to run POE devices over the network cable (low change, other than to APs).