I have few subdomains configured for my domain (which is managed via Google Domains).
only the sub-domains i have configured load a page, any other non-configured sub-domain results in a standard "page cannot be found" style error - which is the expected outcome.

previously i have using Ubiquiti Edgerouter X to manage the updating of my external IP to via the dyndns service to google domains. this entailed adding an entry for each sub-domain the edgeOS config (with the appropriate user/pass), and this updated just fine.

when trying to achive the same thing within DSM External Access > DDNS it seems i can only add 1 entry per Service Provider (where google is listed). this means that i can only updated 1 of my subdomains and the other 5 can't be. surely this can't be correct?

Unfortunately this is true. One updater per listed DDNS service provider. If you can create an usable custom 'provider' for Google then you can make as many as you need for the sub-domains.

There may be a Docker way to do what you want.

well there is an API URL that can be used to update each sub-domain, so maybe that is an option?

That's what you can use to formulate the custom provider via Customize button.