At the moment I have set up my old Draytek 2860 as an access point connected to the supplied Fritzbox router from my service provider. I have connected it via ethernet and it's in the lounge behind the TV, I then connect several devices to the ports on the Draytek. I also use the 2860 to manage guest wifi access. Would I be able to do the same with either the 903 or 906 and would there be any benefit apart from the APs being smaller?

The wifi generation will be newer (ax) with a 906, and possibly newer (ac) with a 903.

Other than that you'll not be gaining much, if anything, really.

I'd certainly remove the AP906 from your list of potential options; it deosn't seem compatible with many other APs and isn't mentioned as a product on draytek.com so I get the impression it's a dead-end product.  See this thread for more info:

https://www.draytek.co.uk/forum/wireless-lan/25586-warning-mesh-compatibility-between-aps-not-as-id-have-expected-it-to-be