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Looked into an alternative to HTPB: PBAN (used on NASA boosters). But PBAN is more difficult to mix, and has similar performance. (https://www.nasa.gov/returntoflight/system/system_SRB.html) (http://www.braeunig.us/space/propel.htm#:~:text=PBAN%20formulations%20give%20a%20slightly,more%20flexible%20than%20PBAN%20binder)
We can decrease burn rate by including a suppressant, such as oxamide (which should not impact performance). (https://www.nakka-rocketry.net/burnrate.html)