I have some questions, first:
1. If you're worried about a shortfall, why are you trying to apply as early as possible, and use priority? Why not let the system take its time, buying you time toward ILR?
2. Also, why was your first FLR(M) issued for less than 30 months? Why didn't you notify them when it was issued?
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Now, I don't know the answer to this, but is there any chance that your time on the "Parent" visa counts toward your five years? Ksand24 is likely to know the answer to this.
Assuming that initial period does not count toward the five years for ILR, your 5 year anniversary from the first FLR(M) is July 27, 2022. 28 days before that is the earliest you would be able to apply for ILR, so you need your next visa to last at least until 29 June, 2022. Subtract 30 months from that, and you want your next FLR(M) to be issued on or after 29 December, 2019.
If you submit your online application in the first week of December, just before your current visa expires, then book your biometrics for 3 weeks later (as is probably going to be necessary), you're nearly to 29 December, already, and they still need time to process it and make a decision. I still probably wouldn't do priority, in your case. But the timeline can work. Just.