The FLR(M) is supposed to start from the expiry date of your previous visa, but it may not... so I would not risk attempting to apply early, just in case they make it valid from when they issue it.
If the latter does happen, you could find yourself in a situation where your FLR(M) expires several months before you qualify to apply for ILR... which would mean spending another £2,000+ on a second FLR(M) and IHS surcharge to bridge the gap before ILR.
Of course, if it is the case that the online application won’t let you apply earlier than 28 days, as you said, it’s a moot point anyway.
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