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.

Further to the above, I've now tried aggregating ports on a P1281x switch (used for an optical connection to a switch downstairs) for connection to aggregated ports on the 2927ax.
(It's possible to create static aggregation on the P1281x)

I've followed the instructions here:-  https://www.draytek.co.uk/support/guides/kb-vigor-router-lag

This appears to work OK when set up, BUT when the router re-boots the link is not re-established. So basically useless, unless someone can suggest how to fix this?

Update:-  determined not to be beaten by this, I've been trying again, even factory re-settingt the 2927ax and starting again.  Weirdly, sometimes when I create the aggregations and connect the second cable, the P1281x detects a loop and shuts the ports down, although I cannot fathom where the loop is being created.  At other times it works fine until the router is rebooted, when it behaves as above.

It appears from this post https://www.draytek.co.uk/forum/routers-and-firewalls/25740-vigor-2866ax-issue-with-fw-4-5-0?start=12#105668
that Draytek are aware there might be an issue with Link Aggregation...
Since 4.5.1 is supposed to be a critical update I'm loath to revert to the previous one, so maybe just wait for the next one and see if they fix the issue...

I tried reverting briefly to 4.4.6.2 - made no difference, as soon as I connected the second cable to the 2927 the switch detected a loop and shut the ports down...