FLDIGI does not use OmniRig. For FT8/JS8 there are other options than OmniRig, so you can go without it.
For HDSDR however, I am not aware of any other serial CAT rig sync control for HDSDR other than OmniRig (but perhaps here) if you need to sync HDSDR also.
The CAT settings in HDSDR call the OmniRig config program, so that is the same for all programs using OmniRig. Here is my OmniRig settings file (using 9600 baud) where you can see the OmniRig seetings. HDSDR stores its settings in the Windows Registry (not for the faint-hearted), a lot of technical variables.
But my HDSDR CAT settings in Options F7 are:
1) CAT to Radio -> sync Rig1 + sync to Omni-Rig + sync from Omni-Rig + sync Tune frequency + sync Modulation (=mode).
2) Misc Options -> Tune fixed to 'LO<->Tune Offset'
3) "set LO <-> Tune Offset" to -60000 (=60 kHz local oscillator offset).
Anyhow, if you run more than one program with CAT, you will need VSPE so they can share the rig (enhanced) COM port, even if it said not always work ;-). Note: all OmniRig-configured radio programs together count as one in this respect.
To me, OmniRig is the robust CAT solution if the radio programs offers such a possibility.
As I said, with VSPE I can run FLDIGI, AClog, HDSDR (with SDRPlay RSP1a) feeding a CW Skimmer, and optionally WSJT-X or JS8CALL (they have OmniRig as one rig "type"), at the same time. Not using all of them at the same time, but all follows frequency and mode beautifully - change the rig VFO or mode, change freq/band/mode in FLDIGI or CW Skimmer or HDSDR - and all other follow nicely.
GL & 73 de SA7CND