Comment by atmosx

> My main question is: Is there any method or configuration that can ensure that even on my self-hosted VPS, the provider cannot access my data?

Short answer: No, you cannot.

Long answer: It depends on the resources they plan to allocate on the task. This article provides an excellent deep dive into the topic, based on an fairly recent incident involving an XMPP server in Germany: https://www.devever.net/~hl/xmpp-incident