Have you looked at the Sigenergy Sigenstor products? I just had a solar system installed along with that, which includes whole-house backup.

Their gateway product (mine is the HomeMax TP AU) is perhaps most relevant here for what you're talking about - it supports input from multiple inverters (so you could retrofit now, and add more inverter capacity later), and you can choose to route full backup (from an inverter) to your main panel, or to a sub-panel with another non-backup output feeding the main panel. From what I gather, it wouldn't be horrendously difficult (all things considered) to swap that up down the line, should you increase your storage capacity (fairly standard AC electrical panel work).