I've done it before with starters - good ones - just knowing that I can't take any more than 10-15% of their rated capacity out of them each cycle. So it can be done, but is just not advisable.
The total irony here is that when you draw such a small capacity out of them, they are relatively easy to get recharged via solar.
The load tester will only go so far. Speaking of which, trying to do a typical C/20 capacity test on starters really puts the hurt on them, especially if used, so if you do a capacity test, only take it to no more than 50% DOD and scale your results! Maybe do only a 10-hour test at the C/20 rate, and double the measured capacity.
Oh yes, AGM are deep cycle - but there's a big catch! You can't do it too often! Ie, that capability is only there for the off-hand random deep discharge. But on a regular cyclic basis - no, not unless getting only 100 cycles max out of them is ok. $$
So the natural progression is from starters to so-called "dual purpose" or "marine" deep cycle. They are less acidic than starters, but have the same thin plates. So they now may get only 150-200 cycles from deep discharge. Same deal. Ok, maybe at the most a 20% DOD per cycle if you are going to use them for solar.
TRUE deep-cycle batteries have thick plates, and are designed for long-term power draw.
Still, there's something to learn here. An Optimate charger discussed above will help set up those starters the best they can be. Far better than just a simple vehicular load-test, which is no longer your application with these.
If those batts are healthy, and you draw no more than 10-15% from them per cycle, and they get recharged properly, you
may be able to sneak in 3 to 4 years of use.