Prompted by discussion elsewhere I thought I'd try connecting my 2927ax via two aggregated ports (2 and 3) to two aggregated ports (7 and on the P1092 switch.  According to my (re)search this should be possible, although the settings on my switch (both devices on latest available firmware) do not seem to correspond to those I find on the web, where reference is made to choosing IP addresses - I don't have anything like that.  Having set up the aggregated ports on both devices I proceeded to connect one cable, whereupon all was fine.  However, on connecting the second cable both the router GUI and the switch GUI became inaccessible, although the PC still claimed that it was receiving its correct IP address from the router.  Disconnecting the cables made no difference.  The only way back from this situation was, after disconnecting them, to power down and re-boot the router.
I ssupect some sort of odd looping may be taking place, but I don't see why.  The two aggregated ports on the router connect quite happily to a Synology NAS.

This is the aggregation page on the switch  

Any suggestions appreciated

ah - I see from this article https://www.draytek.co.uk/support/guides/kb-vigor-router-lag
that the router doesn't support LACP, which appears to be the only mode available on the switch..
so that knocks that idea into touch....

…that’s what I was alluding to in the other thread. Shame they don’t support it.

On the 2962 model, there is a 2.5GbE WAN port, set to WAN by default but it can be changed to a LAN port, which I did to increase the inter-LAN bandwidth between VLANs communicating via the router. Then just used the LACP features of an external switch to max out the bandwidth of a few LAGs to 2x NAS boxes and a backbone LAG to another switch much further away.