Managed a fairly large matomo site in the past. Using queue plugin (https://plugins.matomo.org/QueuedTracking) with Redis Cluster really improves the situation. We actually built a custom plugin with Nginx + Lua to avoid PHP altogether for the tracking part. Scaling ingestion then wasn't the problem, draining the queue was

The tracking was not the issue the problem was report generation with segments. Every segment makes you regenerate all the reports. Tracking part is a problem because you need to split the tracking and report part of you want to have something robust.