Battery size/type depending on desired usage
This battery (Yuasa NP18-12, 12V, 18Ah) should be better suited for your needs.

If you look at the "Typical discharge characteristics NP range" graph in the datasheet (last page, bottom right), you'll see that the battery loses most of its capacity within just under 200 cycles if you discharge it completely in every cycle. This is exactly what's happened to your 7Ah battery - you ran it at almost 100% depth-of-discharge and it lost a lot of capacity in about 100 cycles.

The key to a long-lasting battery in your application is using a battery rated for cyclic discharge with way more capacity than you actually need. The NP18-12 is such a battery. In your case, it will run at 30% DOD, so it should last for more than 1000 cycles or about 3 years according to the datasheet.