In absence of other internet connection, does it make sense to practice configuring RB5009ÙPr using...

a mobile phone hotspot shared to RB5009UPr via Windows 10 laptop RJ45 port?

I am waiting for a optic fiber connection later this year. but I will get RB5009, Ubiguiti unifi lite 8 POE+ switch and U6 long Range AP, hopefully later this week.

My plan is to power U6 Long Range via the switch. Would there be any benefit in powering the U6 LR using a separate POE injector and connecting the AP directly to the RB5009UPr?

If that switch behaves as it should, then there’s no performance benefit in doing what you wrote (and I quoted).

Thanks, so it is just a question of me digging into how the switch, router and AP are properly configured for a smooth co-operation.
I also take there is no reason why the internet access provided by my laptop windows 10 PC could not be used for testing different configurations of the setup.

When I wrote “If that switch behaves as it should” I was thinking about possible effects one additional switch in packet path might have. E.g. a sligtly increased latency (normally we’re talking about fractions of a millisecond). A misbehaving switch could corrupt frames, drop frames, delay frames, etc. When it comes to powering those APs, using a PoE switch can mean better power efficiency (single power adapter instead of quite a few) but can be worse power efficiency (losses on PoE electronics, etc). And so on. But as I wrote, generally it should be OK.

And networked gear generally doesn’t care which exact way packets take to reach remote end of connection (if the properties of the link adhere to certain minimum levels … IoAC might not fit the bill). If routing is done right, then your Windows 10 laptop (with broadband modem built in?) would be just fine.

My Windows 10 laptop connects to internet via a smartphone WiFi hospot.

Let me explain in detail → Under Windows 10 Control panel → Network and Internet → Network Connections, I select the active smartphone Wi-Fi connection properties → Sharing → Allow other network users to connect through this computers internet connection.

Then I connect my router to the Windows laptop ethernet port and the router is then connected to the internet via the Windows 10 laptop / Smartphone hotspot.