[parenthephobia]

I was involved with running a SaaS which also had issues with free tier signups.

Two observations.

1. Prioritise paying customers. Use spare provisioned capacity to provide for the free tier: don't spin up new capacity for free tier customers - or at least do it on your own terms. Now it doesn't matter if people milk your free tier unless you believe, almost certainly incorrectly, that these people would pay if they couldn't sign up for more trials.

2. We had repeat signups from gmail accounts, but almost none from disposable addresses as such. Any service that lets you have multiple email addresses for free is a potential "risk". That obviously includes any self-run email server, or any corporate email server from the perspective of a user allowed to add new addresses to their mailbox.