I do not know of a web portal for Signal.
The Desktop application (Electron) does not yet have proxy support according to support.signal.org/hc/en-us/articles/360056052052-Proxy-Support
It seems that you are struggling with many problems using:
- Tor (captchas, 403 error messages)
- Tor Browser (web applications like Instagram)
The first problem comes with the security settings that website owners choose and there is little you can do about it on your end.
The second is related to the aforementioned incompatibility of limiting browser APIs to avoid fingerprinting and the web applications that expect them, do not fail gracefully when they do not work as expected, and bundle functionality with tracking.
My solution is to not use these web applications. Of course, it is not always possible. So, if I have to use them, I use disposable sessions (similar to what Tails does). If I am forced to use them for a longer period, I dedicate a virtualized system completely for this purpose, or even a physically separated device.