We tested four privacy-focused email providers that offer CalDAV. Fastmail works with all three apps. The others range from partly usable to not usable at all.
Can I use Spectacled with my Google account? With Outlook? With iCloud? The short answer is no, and we would rather you found that out here in two minutes than after an evening of trying. The longer answer is worth reading, because it explains what Spectacled actually needs from a server – and once…

Radicale works with all three Spectacled apps with nothing to configure on either side. One thing to know if your server lives on your own network.
All three Spectacled apps work with Nextcloud without any special configuration. The one thing worth doing first: generate an app password instead of using your account password.

We tested all three Spectacled apps against Baïkal and found no issues. The one thing to know beforehand: switch WebDAV authentication from Digest to Basic.