I know you said you want to get away from SLA, but at the same time it's not difficult to do it with the correct battery chemistry.
Earlier this week, I rebuilt mine with this
https://www.amazon.com/Battery-replacement-QB24-QB26-QUANTUM/dp/B002BJZ2XI
It's a 3 pack of smaller versions of the same SLA cells as used in the original battery. It took me a little bit of finagling and some extra foam inside the case to get it to all fit and line up correctly, but once I did it worked perfectly.
One of the issues you're going to run into with any alternate chemistry is that you really need 6V+. A single lead acid cell is 2V, give or take, while most common rechargeable chemistry these days is 1.5V. That means that you're going to need to pack 4 cells into the pack to get the correct voltage. It's hard to beat lead acid chemistry for power density, and I don't know how you'd get out of an NiMH battery. The original cells were 5AH. The only NiMH cells I found in a quick search with that rating were 1.2V, which means that you're going to need 5 in the pack to get close to correct. That's going to be a tight fit for a C or D sized cell.