How about:
...Swi|tch 1 Switch 2 Switch 3 Swi|tch 1...
| p42 =LACP= p40 p42 =LACP= p40 p42 =LACP= p40 |
| p43 =LACP= p41 p43 =LACP= p41 p43 =LACP= p41 |
Plus RSTP...
Its what I had thought of doing between my leaf switches (only have a single DGS-1210 as my core switch)... except the (other)-Link switches don't do STP...and only static LAG.
Strange that on my DGS-1210, I had to enable LBD, in order to disable it on the ports, before I could setup the LAGs The leaf switches have an LBD feature, but no per port configuration...and its only port based.
LK