I'd ride the 31 hard, daily, get 15.0V in it every sunny day that you can, if you have enough solar to do so, and put things on the AGM 1x a week while you massage the 31 to a full 16.0V Equalize voltage work out, or what ever it takes to max out the V it will take, then put things back on the 31 and top off the AGM at whatever voltage the mfg recommends once it is back to starting duties.
If the SG drops too much every 7 days... do the 16.0V thing sooner, or get rid of the 31 once it becomes unusable to you, which may be sooner, rather than later.