I kind of think the ntp driver could handle this in an elegant way.

have a driver at 127.127.20.20 -> nmea time

make it normal stratum ntp, like stratum 1 or 2

doesn't give out the time unless there is a position fix (normal behavior)

and make another driver 127.127.20.99 -> nmea time without position fix

at the start, there is no time given at all

then you might get a time without a position fix

then you get a time with a position fix

then you might lose the position fix, but still give out a time.

make it a stratum 14 ntp server