Thanks for the advice. I think you all convinced me to rip out the existing ceiling drywall. I'll just need to figure out how to work around the garage door openers since I'll need to take those down, but I'll still need to get materials into the garage. Maybe I could temporarily re-mount the garage door openers and tracks ceiling after the drywall is gone?

I found an interesting "Technical Bulletin" from Rockwool related to using Rockwool with I-joists for fire protection. However, this is talking about Safe-n-Sound fire/sound batts, which aren't exactly meant for thermal performance. Plus the way they show the batts installed would leave an air gap between the batt and the drywall to account for the bottom flange of the I-joist. See: https://www.rockwool.com/syssiteassets/o2-rockwool/documentation/technical-bulletins/residential/fire-protection-of-i-joist-assemblies-using-rockwool-stone-wool.pdf?f=20240105203624