OP has pixel and is in the +1 area code from the picture meaning they are in America. So I don’t get why he/she is not using Google Call screen.
I still get spam calls, but my phone never rings or alerts me. The only way I find out is by occasionally opening call history. The assistant can recognize most spam automated messages and it automatically hangs up. For real spam callers; they mostly hang up when they realize they have to talk to a robot first.
Not open source, but I use the Google TV chromecast in “apps only” mode.
Its very minimal but you still get a single picture of whatever show is being promoted that week. No other intrusive adds have been added {yet}.