Is the footage you are recording from your URSA BMD Film ProRes or DNG raw?
This is speculation, but if you are recording raw, I believe that Resolve does not use a single IDT for DNGs from BMD’s own cameras, but rather decodes the raw directly to ACES colour space using a matrix which is dependent on the shooting white balance.
The IDT for BMF Film ProRes has no knowledge of the shooting white balance, so I expect a “one size fits all” IDT is used. Your approach of putting a CMS image through the fixed IDT + RRT + ODT to create a LUT is a suitable one to match the fixed IDT. But it will not match the variable IDT for raw. Or at least it will only do so at one particular white balance.
But I repeat, this is my speculative theory. I have no inside knowledge.
Regarding my DCTL scripts Willian refers to above, I don’t think they will be of any help to you. They can be used to create a LUT which does not include the RRT or ODT, for use in e.g. Nuke, using a method described elsewhere on this forum, but the LUT you need should have everything (IDT, RRT, & ODT) baked in, so your CMS image approach is a good one.