firewalla VS RPi-Monitor

firewalla is open source https://github.com/firewalla/firewalla

Just to follow up because I think I misread OP—they’re looking at Firewalla who publishes their source on github, and it looks a lot like your classic FOSS firewall running suricata and probably doing DNS-based filtering. I bet if you dig into their source you’ll find dnscryptproxy, unbound, or dnsmasq with filter sets.

Using the Firewalla VPN Client for a 3rd party connection is one way. What this means is that devices on your network can initiate connections through the VPN to external addresses, and those external addresses can respond and make use of the established connection (ex: your machine uses a browser to request the webpage https://help.firewalla.com through the vpn connection, and the server hosting the webpage receives the request and is able to respond, by sending the requested webpage)

search https://help.firewalla.com you will find more articles on this topic. And if not, the communitly likely have more https://help.firewalla.com/hc/en-us/community/topics/360001948014-Expanding-Firewalla-Docker-Third-Party-Apps-Scripts-

there are a few threads https://help.firewalla.com like this https://help.firewalla.com/hc/en-us/community/posts/4411622449811-Omada-Controller-Docker

From my understanding zeek does the inspection (probably not https / tls ) and generates alerts (internally) which Firewallas software (https://github.com/firewalla/firewalla) picks up and analyses then throws out alerts / blocks as they see fit.

Technically yes, but also maybe no. Firewalla's software is open source. You can download and modify it on GitHub. If you really wanted to, you could fork it and strip out all of the firewalla-specific code (ie licensing, app integration, etc.) and replace it with your own code.

Rpi-monitor

I found this: https://github.com/XavierBerger/RPi-Monitor/issues/199

Have a look at Rpi-Monitor as it has a number of monitors out of the box and is easily extensible if you want to add additional ones. It would have helped identify your increasing memory usage over time.

I really like a monitoring tool with web UI like RPI Monitor. Ex: screenshot

In case you have the system in a SD Card, I will also recommend to install log2ram (https://pimylifeup.com/raspberry-pi-log2ram/) . A hardware monitor will be interesting, like rpimonitor (https://github.com/XavierBerger/RPi-Monitor) and last but not least, you can add a WIFI / LAN intruder detector with Pi.Alert ( https://github.com/pucherot/Pi.Alert)