I apologise in that I am a bit late to the party and your issue looks to be resolved, but I thought I'd chime in if somebody was to read this thread in the future.
When I bought my Jazz Hybrid, it was standing at the dealers and the battery was completely shot, so after some research, I bought a regular
YUASA YBX5054 (40Ah / 340CCA) battery for £46.91 delivered, they are about that price now. I can report that I have had absolutely no issues with the battery since installing it more than 12 months ago. AGM batteries are dear and I don't think you need one.
And in terms of battery maintenance, for as long as I can remember, I have always hooked up each one of our cars to a car battery charger and re-conditioner, once every quarter, for an overnight charge session. The result is that our oldest car, an old diesel from 2001 which we bought in 2010, is still running its original battery with no issues and this is not a car that gets used much at all, maybe 3,000 to 4,000 miles a year.
In my view, the Swedish company CTEK make the best battery chargers. I have a CTEK MXS 5.0, which sells for about £65 and slightly less if buying from eBay. It has saved me several batteries and even resuscitated batteries I assumed were paper weights.
So I really do advise, if you can afford to splash out, get yourself a CTEK and especially in cold harsh winter, stick on the battery overnight a couple of times.
CTEK website:
https://www.ctek.com/uk/education/car-battery-chargers