There is no concept of "float charging" for li-ion.
You can apply (current-limited, i.e. CC-CV) 100% open-circuit voltage, which would be something like 4.17V for a typical 4.20V stop-at-C/20-rated cell, and nothing special really happens, even if held for years. The cell stays at 100% which of course means accelerated calendar fading compared to, say, 50% state-of-charge, but this is not grossly different from micro-cycling the cell between 80% and 100%, which could be indeed worse.
Float charging is a thing for chemistries like lead-acid, NiCd or NiMH, which can be held at a higher than 100%SoC open circuit voltage and they shunt extra charge in non-damaging (or very slowly damaging) chemical reactions, producing heat. These cells actually consume significant amount of current when floating, also allowing them to self-balance in series string. Equivalent use case would be floating a li-ion cell at like 4.6V or so, the difference is just the reactions are irreversible and destructive so you can't do that.
So in essence, "you may not float charge li-ion cells" means "it can't be overcharged unlike some other chemistries". It does not mean you cannot apply say 4.15V continuously.
But as always, you need to know what you are doing, and do it correctly.
The 4.35V BMS is only for last resort protection, you need more accurate voltage regulation and for best lifetime, I suggest going as low as 60% SoC, so somewhere around 3.8-3.9V, if you can afford the wasted capacity.