Thanks @psherman. Maybe I can check my thinking with you to see if it makes sense. On my downstream router I have stubby set to send DNS requests out via CleanBrowsing Family Filter. Now this works great, but I've noticed that on my Microsoft Office 365 programs sometimes the 'Exchange Front Door' is getting set to an undesirable location (Germany). So my thinking was to make an override for the Microsoft domains on my downstream router that means those domains get sent out to the upstream router (and hence via my ISP's DNS servers). CleanBrowsing is there to block offensive content, but for Office365 I care about getting the best possible routing, and I think my ISP's DNS, Google or Cloudflare may be better for that. Does this seem reasonable?
I see that such selective domain-based DNS forwarding is facilitated in OpenWrt:
@dave14305 I've marked your post as the solution (many thanks!), since this made the requests work (albeit I am still unsure about whether what I have in mind above makes sense). I'd also value any thoughts you may have on the above.