The powertanks are tempting, and can be handy to rescue a car with flat battery, but for much longer life/value, suggest next time something like:
https://www.ebay.com.au/itm/12V-33AH....c100005.m1851
+
https://www.ebay.com.au/itm/135AH-12....c100005.m1851https://www.ebay.com.au/itm/135AH-12....c100005.m1851
+
https://www.supercheapauto.com.au/p/...id=SCA01060203
Getting up towards $300, but much better utility and grunt versus 2 powertank/jumpstarts.
Deep cycle AGMs are designed to handle periods down to 33% full charge and should last many years if recharged properly in timely manner (3-10 years).
SLAs in powertanks/jumpstart are designed to put out high amps for short period, but don't generally last anything like the time of an AGM/deep cycle and are happiest staying near full charge at all times (think back up battery for a house alarm for example - 2-5 years typical lifespan).