Shared frequencies for digital simplex is fine due to the different modes not opening the squelch as one would if using 146.520, or 446.000 MHz, which should only be used for FM analog obviously. Thus, digital simplex modes could use agreed upon frequencies for VHF and UHF to keep things friendly. Coordinating bodies should recognize this need and help guide everyone to an amicable environment. There's plenty of room for whatever digital mode to share these frequencies and not be bothered with interference 99.9% of the time.
These should work fine for the Northern California area which NARCC coordinates.
This is approximately Bakersfield north to the border.
433.500 MHz - 1
441.500 MHz - 2
145.680 MHz - 1
146.580 MHz - 2
Using the below defaults will allow one to TX/RX that same mode with others, or adjust as needed for reduced traffic.
P25 simplex ( NAC $293 )
DMR simplex ( TG99 / CC1 / TS1 /Admit Criteria: Always / In Call Criteria: TX or Always )
D-Star simplex ( MYCALL = your callsign) / URCALL = CQCQCQ / RPT1 = blank / RPT2 = blank )
Yaesu Fusion simplex/GM ( Group Monitor ) Use DG-ID of 00
(DN = TX Callsign & GPS data if available)
* For C4FM on HF perhaps use 29.250 MHz and 51.500 MHz (DN)