There are quite a few things to consider here. One not to underestimate is personal preference: I personally find it highly difficult to design and develop for a platform I dislike - like Android for me. There are just so many places on Android that I feel are a bit ugly (again, personal preference). Anyways, this leads me to not feeling comfortable using Android so I couldn't deliver a great product for that platform.
That out of the way, don't underestimate that learning a second language can be quite a task, especially if you are still learning the first one. Going with things like Flutter that provide multiplatform support: These are often either missing features/APIs or not delivering a great user experience. Same goes for web apps and such. If you are aiming for a great user experience and a high polish, native development would be the way to go on each platform.
Do you want to earn money with that app? If not, there is nothing to lose. If yes, you may get disappointed as based on various studies Android users are less willing to spend money for apps than iOS users - not to mention piracy and sideloading. So even with an even market share of both systems for that particualr region, revenue likely won't be the same. I don't think you are aiming for financial success with an app for a single event , though.
I personally would focus on one platform and try to figure new app ideas to build on - and grow - your skills. If you want to use Firebase you can still do that for an iOS only app, so that you then already scored that skill should you go multiplatform later. Also iCloud - as much of an Apple guy I am - can get a bit unreliable at times.
