I'm not in front of my main computer at the moment so I can't give you perfect step by step instructions (maybe a kind soul on this board can jump in and do it?) but if you have a Linux computer available, say a Raspberry Pi or similar, it should be straightforward if you spend some time on it.
There's an install guide for it here:
https://github.com/atauenis/webone/wiki/Linux-installation
The steps seem to be
1. Download the install package
2. Install some dependencies
3. Edit the config file
4. Start it
Then, on your classic Mac, in your browser of choice, you'd set the IP address of your Linux computer running this thing as the proxy server and browse away.
Any website you visit with modern security protocols will get processed by the proxy server and it will serve them to your classic browser in a way it can understand.
Classic computers are a bit like classic cars, if you want to entertain that comparison. You need to put in a bit of effort to keep them in good working condition, and you'll find that as time moves on, certain things may not work as designed anymore. For example, radio is largely going digital and the analog stations will be switched off at some point. Then you need to install a converter box so your old analog radio can still get the digital stations. It's a similar story with old browsers and TLS in this case.