Typical use case of a SLA battery is gate operation, with it living almost the entire life on float charge, with only small draws due to gate operation. Similar to your application.
I just had the gate motor battery die, and put in a temp one, and took the 3 year old deep cycle one out and, after some not so gentle work, got the sealed covers off, and saw it was simply dry. topped up the water, and charged overnight with the close to 70 year old charger I use for these, and it recovered somewhat voltage wise to 13.8V resting.
Then, lacking a load tester, I did a DIY version, using some nichrome sealing element strip, around 10cm long, to do a 20 second discharge ( melted the wire after the 20 second mark, it was glowing white) and it did not drop below 12V on the terminals, so I guess the sulphation was not too bad.
Left it another week, and also stuck the tops back on, and checked, and it was reading 12.6V, which was good enough, as I had not recharged it after the load test. Took it and put back in the separate battery box ( original used a 7AH cell, and there was no room in the housing for both the 7Ah battery and the charger unit, so separate it was, plus there had to be wiring in there for connecting stuff), and it can gas off there all it wants in the galvanised steel box.
All I want is another year or so of operation out of it, and then I will not put in a special one, just a regular car battery, and see if there is any life difference, as the cheap automobile battery ( around half the price of the deep cycle) I just peel off the sticker and open the cells, to refill the water when needed. 90% of car batteries fail because they vent off water, dry out and sulphate, seems that checking the battery water is no longer encouraged, while it is a very cheap thing to do and will greatly increase battery lifetime.