Off-grid backup power using hybrid inverters?

[Sanwizard]

Sounds like you have researched the market fairly well. I will leave you with a couple of additional comments that may be of some help.
1) Any hybrid inverter can be used as a whole house solution but it does require a Line Side utility tap between the meter and your main panel for AC Input to the inverter and then the inverter AC Output is used to power the entire main panel. I'm using a Schneider XW+ that powers our main 200A panel via a backfed 60A 2 pole breaker. I have also installed a maintenance bypass transfer switch such that if the inverter fails I can feed power from the meter directly into the main breaker on the main panel while the inverter can be completely isolated by turning off both the input and output breakers.
This is a new product from Siemens designed specifically for a Line Side Tap. https://connectder.com/

2) Perhaps the Franklin Whole House back up system would be of interest. I believe it is a bi-directional battery/inverter so only needs one 2 pole breaker for each unit. https://www.franklinwh.com/

Do you have pics or a wiring diagram of your implementation? I am looking to use the existing SMA inverters and panels to charge the batteries during the day for two MPP off grid inverters, with the off grid inverters ac coupled to fool the SMA inverters grid source.
My ignorance is around how to tie the MPP's into the main.