Hi Jay, is there any particular reason you have them in bowls instead of a tank? 1.5 gallons is very small and the water quality can fluctuate greatly in small volumes. It's good that you are concerned about the ammonia levels and are monitoring those.
Bettas can breathe atmospheric air and one problem with bowls is that the air exchange isn't as good is in a non circular tank, and a covered tank vs bowl keeps the air nice and warm and moist for the fish. Bettas are also tropical fish, not cold water fish and so do benefit from a heated tank.
Marimo balls can be used in cold water. I'm unsure how much they'd reduce ammonia.
Realistically, a filter established with the right bacteria would be an easy way for you to keep the ammonia levels down, however, I understand these aren't really possible to have in a bowl.
The ideal for you and your fish would be to get a tank with filter and heater. If, however, you really can't get a tank for whatever reason (even one of a similar size - although bigger would be better and easier for you to maintain) then you can increase your water changes more times a week, even every day. If you do very regular water changes, every day or even twice a day, then the water quality of your betta's water vs the new water shouldn't be too different and so complete water changes are possible (provided the water is de-chlorinated). However, bear in mind that netting a fish out of water is very stressful for them so best to avoid that (therefore a true 100% water change is not possible).
Have attached an old picture of the tank I had for my betta in 2011, this was a 5.5 gallon tank and I did 50-70% water changes 2-3 times a week depending on how much time I had, this was filtered, with a heater and planted. He had lots of places to hide if chose at the back and the front of the tank was open swimming space. This was how I liked the look of the tank at the time, and never had any problems with my betta.