Ok batteries come in assorted types. For this
Starting: These are batterie which generally are not all that "Big" Capacity wise (IE: 70-80 Amp Hour) but capable of very high (hundred of amp) current. These betteries need to be kept fairly full 80-90% SOC. run 'em down to 50% and its time for a new battery.. As you found out.
MARINE/deep cycle... Very much a starting battery but you can go a bit lower on charge say 70-80% again run 'em down.. well you found out.
DEEP CYCLE.. The most popular DEEP CYCLE is the GC-2 this is a six volt GOLF CAR (GC) battery you use two of them in series to make a 12 volt battery They don't mind 50 percent. They also hold over 200 amp hours... But they do not like "HIgh" current (Defined as a percentage of the 200 amp hours) so for a Group 27 (about 100 amp hours) 200 amps is HIGH current. for a pair of GC-2 it is not.
BUT THe GC2 stands a much better chance of recovery from an OH S!!! level discharge (below 50%)
THey also make a GC-12 12 volts. about 150 AH as I recall.