I'm sorry you had a crap time, and agreed that in the past things were not great. But in defence of the Matrix team, we fixed almost all known encryption in ~Sept 2024, and the new generation of clients fixes the slow sync issues.

In terms of message retention: https://element-hq.github.io/synapse/latest/message_retentio... is how you should have/could have cleaned up those rooms. (It's not exposed in Element's UI yet as we've been prioritising more fundamental stuff).