Did you weigh home screen widgets here? For "log mileage when you refuel", a one-tap widget feels much closer to the moment than open Safari, find the bookmark, hit the button. PWAs on iOS still can't put one on the home screen. Curious if cross-platform was the call from the start, or if widgets came up at all.
Widgets are possible to do with Flutter, but you have to implement them natively and do a bunch of FFI for passing data. I went cross-platform from the start because I have only so much time, and it's much easier to write 95% of the code once instead of twice.
Here's a low-tech approach: an SMS/WhatsApp/Telegram bot. You can just text it the info and even the location. This means all the logic is server-side and nothing on the client.