Heh.. yaah welcome to apple. Uh, to be honest you probably want a mac. Back when I was playing in that sandbox I invested in a second hand mac mini.. it was less than ideal but not unbearable. At the time developing off a Mac meant no access to apple SDK and the iOS simulator, the SDK has all the good device integrations and the simulator is very useful for testing before waiting for a full build on testflight. The path you're treading, based on my experiences, strikes me as a difficult one but maybe someone can chime if the outside-mac environment has improved.

Given your constraint I suggest leaning more towards react native, you can debug/run that locally in linux, and check out subscribing to something like expo / esd to package, sign and send the app to testflight. Could be cheaper short term than buying a mac.

I haven't used this approach myself, but that is the angle I'd consider without the hardware. Orr like others have suggested start with building a PWA and access as a website.

Edit - to be clear I'm not trying to entirely discourage the approach you found. Give it a solid 4 hour attempt and see how it works out then keep going if you get a useful result, that's basically app development.