Is it possible to build an app for my iphone directly from a windows pc with only a free developer account? From what I can tell the options are either buy a license for $100 (which for my little hobby project is too much) and connect it to unity for cloud builds to testflight, or go and buy a mac (which is even more too much) and do some xcode stuff.
Welcome to apple 
I believe the answer is no, not really. I looked into it, and like you, I dont want to be paying apple 100 a year for something i dont really want, and buying a mac after paying a small fortune for the machine i have was also not an expense i fancied.
As i understand it the cheapest practical option is pay the 100 for the right to compile, and buy a used mac mini off ebay or something
You can prepare PWA version for your app! then upload on you host and ask users to add the app/site to home screen! will add and work like normal ios apps!
this is true, but you will be confined to the limitations of web
If I wasn’t so used to Iphones I would 100% be using an android right now, apple is genuinely so hobbyist hostile, I need them to give me one good reason why I need to be fully in their ecosystem to make private apps.
PWA might be a good secondary option, so I’ll look into it! thanks!