Just run the M730.
As for minimum size cog, I’ve never heard of a good explanation, or an instance, where an older RD wouldn’t shift with an 11t or a 12t cog when Shimano’s minimum was 13t.
I’ve often thought the minimum cog corresponded to what Shimano had available in the correct group (road or mountain) at the time and not on the inability of the RD to function.
The RD-M730 SIS 6 speed came out in 1987. The only 12t Uniglide freewheel/cassette I am aware of in 1987 was Dura Ace. Shimano wouldn’t recommend a road freewheel/cassette for a mtb application.
John