The starting problems have nothing to do with the battery. My GP300 has the problem and it plays no role which battery I use. Even with a power pack with which you can start a Tiger tank after it was parked for 74 years, the GG starter makes a whining noise and would not crank.
As said before, thicker cables may help. After I installed them and an additional cable between the iginition coil's ground and the starter bracket, it was better but not perfect. I start it with the kicker first, then when the engine is warm the problem is gone. I can live with that.
My 19 EC 300 has the new ignition system and different negative/positive cables, about three times thicker than the 2018's plus a stronger battery. The problem is gone though I replaced the stronger and bigger battery with a tiny 2018 one.