One issue I've encountered is that I can't seem to enter a Booking Ref or E-ticket number when adding a flight with (+).

My primary feature request would be to calculate and display the layover duration for any 2 flights with matching arrival & departure airports (perhaps even non-matching airports if they are within the same city). Sometimes I have 4 or 5 flights to get me where I'm going, and I often plan extended (full day or slightly more) layovers at one or more locations. It is a mentally taxing process to calculate layovers because of timezone differences but should be fairly easy to do if you're using UTC internally. A secondary feature request would be a way to group flights, such that I might enter 4 flights for a departure itinerary and 4 for the return, and have them labeled as I wish, or at least visually separated. I would also give a vote of support for the addition of am/pm time display, with +1 "day" spelled out (small font?) as well when appropriate (maybe a red-eye icon?)

I see the website is mobile-first which is nice, it's a good start. Are there plans for a full desktop version at some point?
Ideally I'd be looking for a lot of the tripcase functionality like adding hotel reservations, non-flight transit (cruises, ferries, trains, whatever), custom notes, and crucially the ability to generate a PDF of the itinerary for printing. I don't see any of that on the roadmap so I thought I'd ask if such things align with the project goals?

The proposed subscription levels are affordable but if the user didn't intend to fly in a particular month/quarter/half year/etc what happens then?