I suggest you simply move your house circuits to the LFP and leave the existing FLA as a starting battery. Lithium batteries are usually not meant to start engines anyway. Maybe you could add a 1-B-2 selector switch to be able to combine the batteries when charging from the alternator, just keep an eye on the alternator temp and be sure to turn the selector to the FLA if your app says your LFP is almost full. (Just don't go thru NONE and make sure the selector is a MAKE BEFORE BREAK type.) I don't know how much warning the app will give before the BMS disconnects the charge source and I don't know if having the switch on Both will 1) Work; 2) Prevent the voltage spike. But it may be worth researching. I do know you don't want to leave it on Both when the alternator isn't running. Other options are a DC-DC charger or an alternator/regulator upgrade to one that can restrict the power draw and turn off when the BMS says it is about to disconnect. (See Wakespeed WS500 regulator.) I don't think the last option is viable for the small size of your LFP 'bank' due to the expense. I have heard people suggest an ACR to automate combining the banks when the alternator is running, but I don't know if that is a workable solution either.
At this time, I am simply leaving the engine and alternator on a dedicated AGM battery and charging my LFP bank from my solar array and if needed a shore power charger.
@Gene Neill has a Catalina 22 with an outboard. There is no alternator as we who have inboard engines know them. Basically the alternator on small outboards are magnetos with a rectifier, there is no regulator. If the RPMs go up, the output goes up. Given the small output of the OB the battery should be able to absorb all the current the OB is producing. I also doubt he has a separate house and start battery. If he does, the start battery might be as small as a lawn tractor battery.
There may be some risk of a BMS shut down due to an over voltage state which might compromise the rectifiers. If that should happen it could render the OB inoperable, but I don't know enough to render a definitive solution. Certainly pull starting a small outboard is not all that difficult, the issue would be providing power to the spark plug, would a blown rectifier prevent power getting to the plug?
The prudent course of action would be to disconnect the battery from the OB and use the pull start until it can be ascertained that a blown rectifier would not affect the OB's operation.