Ah, simples, send current value with two or more delta values (a byte), like forward error correction (whatever that is), but in reverse.
So 21.3, -0.21 (aka 21.07), +0.1 (aka 21.17).
With the McCloud™️ MegaCalc temperature format, not available in the OptimisedToDeathFormat.com, two decimal places, add 200degC - provides 2dp with range from +455 to -200℃ in just one integer, 16 unsigned bits should be enough for anyone, certainly 640KB is more than enough for anyone.
So 21.3 = 22130, then -21, then 10 - four bytes, current plus last two readings.
Two prior readings usually covers it - you’d be unlucky to lose three packets in a row.
And with TotalRecall™️, if you’ve lost any uplinks, you can always ask (nicely) for a range of fCnt’s to be resent, using the aforementioned MegaCalc calculations. The CleverFirmware™️ will send in appropriate sized chunks. Just make sure you follow FUP on downlinks …