And I tried a bunch... (3m DAC: Cisco SFP-H10GB-CU3M, Flexoptic 10/100/1000BASE-T / RJ45 T.C12.02.A). Now a Ubiquiti OM-SM-10G-D worked.
And probably also only after updating a BPI-R4 I connect with from stock fw to 24.10.5 (SFP hotplugging seemed flaky before that?)