I just want to double check that my math and thought process are correct. I arrived to pick up my BRP on 21/10/2017, so 30 months from that is 21/04/2020. And since we can apply 28 days before 30 months I should be able to get this in before fees potentially go up in April? The only thing I was slightly worried about is I went back to the US to wrap things up until December 2017 so I wasn't sure if I should delay this second visa. We are applying without the 24 hour turnaround, not worth it for this visa (saving it for ILR).
It's a tricky one with the timing, because if you are using the 24-hour turnaround, you want to apply later rather than earlier... because if you apply earlier, you will limit your window to apply for ILR in 2.5 years.
If you want to apply 28 days before you reach 2.5 years, I would NOT use the 24-hour service and instead I would use standard processing, so that you give yourself a couple of months leeway for applying for ILR.
Alternatively, if you really want to use the 24-hour service, I would wait until near the expiry date of your visa to apply.
For example,
The earliest you can apply for FLR(M) is 24th March 2020. The earliest you can apply for ILR is 23rd September 2022.
Say you apply for FLR(M) on 24th March 2020 and your 24-hour service appointment is on 27th March. That means your FLR(M) will be valid from 27th March 2020 to 27th September 2022... which only gives you a 4-day window to apply for ILR!
But if you apply for FLR(M) on, say, May 24th 2020, and your appointment is May 27th, your FLR(M) will be valid from 27th May 2020 to 27th November 2022, giving you 2 months and 4 days to apply for ILR.
Alternatively, if you apply on March 24th 2020, but don't use the 24-hour service and you book your appointment for, say April 20th, and then it takes 8 weeks for the visa to be issued, it will be valid from mid-June 2020 to mid-December 2022, giving you 3 months leeway to apply for ILR... or you could apply even later and get your FLR(M) issued in July or later, giving you 4 months or more to apply for ILR.