Do some rolling and twisting stretches. They're way easier to show than explain but I'll try to help because I had this issue. I was on 8.5 inch rise bars with a topload and just put my 8.66 bars that are a little more narrow and it helped my lower back just right. haha .16 of an inch mattered. Maybe look into going to bigger bars or figuring out how to relax better while on the bike.
Lemme talk you through the main causes of why your back is hurting. One, you probably need to do some abdominal workouts. It REALLY helps your back. Instead of your back having to rely on just itself to hold you up, stronger core muscles will help protect it. Definitely worth a shot dude.
Two. Bike set up. If you have a not long enough frame or too low of bars, that's gonna hurt your back. i'm not on the current with your current setup, so idk, but maybe flip the frontload or upgrade to a quarter inch higher bars. No need to go stupid overboard and go to 10 inchers or something.
three. BREATHING. Breath out when you are landing a trick. Proper breathing while riding helps. I catch people holding their breath when they are riding and landing tricks. You gotta breath from your core if that makes sense. Definitely helped me.
And as for stretched, the touching your toes is only mildly helpful. Breath into your stomach a bit more when you're bent like that and it'll help stretch your back.
Try the rolling stretches on the flat ground. Literally ball up and roll back and forth on your back for a bit. while breathing healthily in and out with movement.
Try to lay on your back and bring your legs back over your head and breath.
Lay on your back and let your waist twist one way and your upper body twist the other and breath. It'll loosen your back muscles up. There are tons of stretches, I'd show you what works for me before riding. I didn't do them one day and i honestly did not ride nearly as well.
Also, drink more water, it's good for your muscles.
References: OneGuyIlluminatiEye, robinson79, Brian Griffin, The Horror Contact, StoreBoughtChild, C_Johnsonbmx, dkTechEthan, etc.