AGM batteries

They're practical for low powered rides or high powered rides that lack range (sprinting). They're really cheap at chromeBattery.com, no affiliation, but you only get 300-400 cycles out of agm batteries so it's worthwhile getting value priced ones made for scooters.

To use 6 agm batteries at 36v (3s 2p) or three in series two columns deep on parallel, with 9ah batteries, would net 18ah at 36v. You could probably ride a good half day on that, but you'll be carrying about 45 lbs of battery, which affects range and the agility of the bike. For 350w at 36v a nice lifepo4 battery is worth saving for, a 20ah at maybe $500.

You could still do it, what I would do is find the highest Ah battery per weight and use three of them in series. You might be able to get the pack down to 25-30 lbs. I use them on my razor dirt bike because they're the only safe affordable battery which can provide enough current. To use lifepo4 would literally cost 10x as much, so my trade off is the weight, which isn't too bad if kept central on the bike.

If you ride on flat ground and your max range is in the 15mi area then 12ah SLAs should work fine. Maybe 10ah with conservative usage.

The SLAs are very tolerant of abuse compared to other chemistries. I can if I want to pull my 5ah Lipos to 3.3v/cell(not ideal) get around 10mi. I cut them off around 3.5/cell and with good management get around 7-8mi. I have to worry about killing a pack with under voltage. Not so much of an issue with SLAs.

12ah SLAs cost in the $20 ea range shipped and will weigh around 7.5lbs ea making a 36v pack weigh in at 22.5lbs. So what if you only get 300-400cycles, that's a lot of riding on a ebike. You don't need AGM. If you design the build to carry the batteries low then the weight is much less of an issue. Charging and general safety is a non issue. SLAs are the most friendly batteries out there.