Hi flucena,
Welcome to the Apple Community ... we are all Apple users, like yourself.
Commiserations! You have been through a lot, and tried many things already with no success.
I am no expert in this area, yet your efforts, and carefully prepared post surely deserves a reply.
Sometimes turning the modem off, and back on works wonders at this end, especially after an ISP has been doing maintenance in the area, and the wi-fi connection has been (seemingly) entirely lost.
_____________________________________________________________________________________
Re: Every browser and app would seemingly get stuck loading without ever delivering an error message.
Wondering if your Mac has plenty of memory / storage space (at least 50Gb free) in order to operate smoothly. ie: How to: Free up storage space on your Mac - Apple Support
If an app on your Mac stops responding, closes unexpectedly or won’t open – Apple Support
___________________________________________________________________________________
Re: I really don't know what to do anymore
Does the wi-fi issue persist in Safe Mode ?
Start up your Mac in safe mode - Apple Support
How to use safe mode on your Mac – Apple Support
If the wi-fi issue doesn't continue while in safe mode,
leave safe mode by restarting your Mac normally.
Then test for the issue again. The issue may have been resolved.
____________________________________________________________________________________
Sometimes third-party (non-Apple) apps (eg: ad-blockers, virus checkers etc.) can cause unusual conflicts or interference.(Virus checkers are un-necessary on Macs, and careful use of un-installers from the app developers can prove advantageous to the Mac OS.)
Also, after an update of the MacOS, some older non-Apple apps may need updating, to be compatible.
___________________________________________________________
Wondering if these Apple Support articles may contain a missing clue, or something you haven't yet tried.
If your Mac isn't connecting to the internet over Wi-Fi - Apple Support
Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support
Solve internet connection problems on Mac – Apple Support
Use Wireless Diagnostics on your Mac – Apple Support
_________________________________________________________________________
Re: it does feel like a serious bug to me.
If you wish you can send feedback to Apple.
They likely won't reply, but if others are also reporting similar difficulties they may make improvements.
Feedback - MacBook Air - Apple
All the best :-)