Had exactly this happen, strong shaking specifically at idle that I couldn't feel while actually moving. Turned out to be a cracked engine mount, way cheaper fix once caught early than I expected it to be going in.

55B24LS is a very common size for Japanese cars in the UAE so most reputable shops stock it regularly. High turnover on popular sizes actually means fresher stock compared to less common battery sizes sitting on shelves longer.

This matches our experience almost exactly, wasted money on retail grade crates early on before switching to a proper manufacturer. The base flexing issue you mentioned was our biggest problem too, night and day difference once we upgraded.