Right, so after deciding all that, I had a reply from spa cycles to my enquiry comparing the GRX chainset with their sub compact chainset as follows.
"Our super compact chainset would be compatible with your bike with the recommended length bottom bracket. You would probably require a front derailleur dropper:
https://spacycles.co.uk/m8b0s106p5161/SPA-CYCLES-Front-Mech-Braze-On-Dropper to accommodate the smaller chainring sizes. The advantage of our super compact over the Shimano GRX is that we can offer custom chainring sizes allowing for much lower gearing and it uses a standard 5 bolt BCD which makes sourcing replacement rings a lot easier. As your rear derailleur is a medium cage it is rated to 11-30 cassette however we have fitted 11-32 with the super compact chainsets and not encountered any issues."
So, my current setup would work as it is, apart from needing to add the dropper, and I'd have the option to go even lower on the gearing in future if required. The only downside I can see to it is I don't know if it's possible to add a power meter to those, I'm guessing probably not. However, it's by far the cheapest way to go subcompact for about £112 and I have the tools to install everything already.