Here's an 18ah Lipo battery designed for 12V applications made by Shorai:
Weighs only 2.12 lbs. Compare that with the PowerSonic which weighs 13 lbs. Note the Shorai has smaller dimensions than a standard 18ah SLA battery and comes with some foam inserts to help with fitment. I might be tempted to make some shims.
It looks like the Shorai has NB connectors. The Lobster Elites use F2 connectors, not sure what the Grand Elites use - but you can buy adapters from any battery shop.
I am an electrical engineer but have not done due diligence on this battery (I have the flu and am about to pass out!). At least you now have something to Google on and see what other people are saying about this battery. I suspect this battery either has a regulator on it, or the per cell voltage of SLA and Lipo are similar enough where it's not a problem. I also would look into whether this battery has a circuit which shuts off the battery if the per-cell voltage drops too far ... it might not be needed if you can get a "feeling" for when the battery votage has dropped too far.
Note you'll have to get a Lipo-specific charger. You'll also need to read up on the care and handling of Lipo batteries - they are different from SLA. Lipo's have very low self-discharge so you can let them sit for long periods of time unused. That means you shouldn't leave your Lipo charger connected during storage.
p.s. If you live in a hot climate like Andre Agassi, or leave your ball machine in a hot car, that might explain some of the life problems with your first SLA battery.