12v battery replacement option

Yes, given your circumstances.
IF.....it will physically fit.
AND.....it has a place to connect the vent tube.

BUT.....then you need to inspect it for water loss every 3 months or so......and look for corrosion in the nearby area.

 

yes, the only issue would be safety in an accident. many folks here use non agm with no issue.

 

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Just about any battery will work, though some will last significantly longer than others.

This thread will show the usable space/dimensions, and some fastening options : Fitting mobility 12volt AGM battery. | PriusChat

Others have even used $20 lawn mower batteries, which may only last a year or so :

I'm using a $25 lawnmower battery for Prius 12v | PriusChat

Battery Replacement - Economy Method | PriusChat

Here is a link to a possible 'group 51' fitment option: 12v battery recommendations | Page 2 | PriusChat

 

This info may be included in the links above, but this is just off the top of my head from some work I've done.

A standard Group 51 can also work (and look almost OEM) if you have some Toyota replacement parts available. I was able to get both the positive battery clamp and the negative clamp/cable from a couple 2007 Camry Hybrids while exploring a junk yard. They are essentially identical to the Gen 2 Prius components, except they use standard terminal sizes instead of JIS. They may be available from other Toyota vehicles, but I've never really looked.