Thanks for your reply.
There is a ISC link between the two 5406zl Switches. On the site of the two 5406 there are two ports configured as dt-lacp. On the site of the 5412 there are two ports configured as lacp.
Regarding "must be the same" firmware version, from my understanding this is only necessary for the switches which are interconnected with the ISC Links.
When I read the actual manual "HPE ArubaOS-Switch Management and Configuration Guide K_KA_KB.16.02" on page 197:
.. The grouped links appear to the downstream device as if they are from a single device. This allows third party devices such as switches, servers, or any other networking device that supports trunking to interoperate
with the distributed trunking switches (DTSs) seamlessly....
I found the information on VSF & DT not supported too, but my understanding here is, that it is not supported on the same switch.
I found no information that it is not supported when I connect two pairs with each technology configured.
Yes, it makes sense to kick the old 5406 switches out and replace this with 5406R and VSF, but we need to replace the 5412 for removing the single point of failure.