Enforcing the use of STIR/SHAKEN across networks and providing an easy way (aka in the messaging and dialer app) to report spam messages and calls would likely

Enforcing the use of STIR/SHAKEN across networks and providing an easy way (aka in the messaging and dialer app) to report spam messages and calls would likely fix 99% of the problem. I'm sure Google and Apple are nipping at the bit to add "Report as spam" in their apps, carriers just need to provide a way to ingest that information and have it be actionable by disallowing spoofing.