I trust google reasonably well when they say they do not scan G Suite (paid) products for advertising. https://support.google.com/work/answer/6056650?hl=en

This is absolutely true. I guess I'm avoiding people who explicitly use my data for profit, in favour of those whose profit model doesn't require it. The trust is that Fastmail et al. are successful enough, and honest enough to not use it.

Ultimately I'd love to host everything myself. But, at present, I don't have the technical knowledge to trust myself to secure everything.

On top of that, on an even more emotional level, the principle of myself being the product doesn't sit right with me.