An update on this.
The feed format now has an offical name: buttwoo :)
For bulk validation I decided to go with the solution Dominic mentioned by just validating the signature of the last message.
It supports subfeeds
There is a PR adding support for it to DB2 and EBT. Both are quite far and just need some box2 work before they can be merged.
I optimized BFE and URI2 recently and that gave a good boost.
Validate + add 15k messages to database buttwoo large: 2.3s buttwoo small: 2.15s classic large: 3.5s classic small: 2.1s