I got a 280Ah pre-built commercial battery and it turns out it's not enough, I'm trying to go off-grid so the daily battery anxiety is getting unfun.

It's a HIGEE 16S300. I want to get another one of the same model, but it's no longer being sold in my area, only the 300Ah model is available.
The shop told me I can't pair a 280Ah and 300Ah together because "they'll be imbalanced, the smaller one will be overcharged while the bigger one is still charging" but I don't believe them, I think they're just trying to get me to buy two 300Ah's.

I'll have to connect them in parallel since the inverter only has one set of battery I/O. Won't their on-board BMS's be already enough to regulate the charging even in parallel? Or do I have to buy an external BMS to measure out current for each battery; if that can work?

I tried looking through the manual and I can only find pic related that might imply that the batteries can talk to each other to regulate charging, but it might be just for monitoring.