My own experience of migrating my calendars and contacts from iCloud to Fastmail went seamlessly. What's more, the user experience on my iPhone went pretty much unchanged, including in Apple's own Calendar and Contacts apps, which has been very nice. Would recommend!

(It's worth noting for the privacy conscious that Australia, where Fastmail is based, has a truly terrible set of laws when it comes to government snooping, so if your threat model includes the Australian government, probably best to avoid Fastmail.)