I just wanted to check when would be the best time to actually submit my application. My visa expires on July 18th. I've heard from some that leaving the submission until later is better, but knowing the amount of time typically done would be really helpful.
This is your second FLR(M), correct? Meaning you entered the UK on a fiance visa, got married and switched to FLR(M)?
In that case, the absolute earliest date you can apply is 28 days before your visa expires... so if your visa expires July 18th, the earliest you can apply will be 20th June 2020.
Normally the issue with applying 'too early' is if you use the Super Priority Service for a 24-hour decision, because that will mean that your visa will be issued quickly and may not give you much time to apply for ILR in 2.5 years.
For example, say you applied online on June 20th, had your biometrics appointment on June 22nd, and your visa was granted on June 22nd... that would mean that it would expire on December 22nd 2022. However, assuming your first FLR(M) started on 18th January 2018, you cannot apply for ILR until December 21st 2022... which gives you a window of only 48 hours to apply for ILR.
However, if you applied online on June 20th, had biometrics on June 22nd, but didn't pay for the Super Priority Service, it would normally take 8 weeks for your visa to be processed, meaning you would get it in late August and it would expire in late February 2023, giving you a window of 2 months to apply for ILR.
ALSO, with the biometrics centres being shut for the last 2.5 months, and no Super Priority service having been available, and the fact that there's likely to be a delay with processing applications since they haven't been able to process any applications from March, April, or May yet, it's likely you'll have to wait several weeks for a biometrics appointment anyway, so regardless of when you apply, it's unlikely you'll get your visa 'too soon' anyway.