I think it's relatively easy to get adapters to combine a flat mount frame with a post/IS mount caliper.
oooh
so like this?
https://www.probikekit.co.uk/bicycle-brakes-and-pads/shimano-adaptor-for-post-type-caliper-flat-type-frame-mount/11211779.html?affil=awin&utm_content=Genie+Shopping&utm_term=Comparison+Engine&utm_source=AWin-259955&utm_medium=affiliate&utm_campaign=AffiliateWin&awc=3977_1595425329_9e20f61287531d14752728ebc346e9ad&variation=11211781]
edit - it's one
@Sainsburys_Ed started
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I think it's relatively easy to get adapters to combine a flat mount frame with a post/IS mount caliper.