I like how the .nl tld in the Netherlands is managed by the non-profit SIDN [0]. They are dedicated to an open internet, and they make sure that I can very easily move my domain between (hosting) providers for example (and indeed it was easy to transfer from transip to vimexx for me). They are truly independent. It's how it should be if you ask me. When I read about the .org issues and now .io, I'm happy with my .nl domain, for personal use.
Hehe, no, at least, not without my credentials (and I turn on 2fa everywhere). You need a key from one provider and provide it to the next, for example Transip (one of the biggest Dutch providers) has the procedure to import a domain outlined here: [0]. To move away from Transip do this: [1]. I bet it's similar for other tlds, not? I have no experience with others. I went through this, no downtime, no issues, DNS was propagated on the same evening I initiated the transfer.
If you are not changing any DNS records (A/MX/CNAME... though you can change the NS records if you move the entire zone correctly, and the old NS servers keep it for a while too), there should never be any downtime related to a domain registrar change.
DNS propagation is not completely under your control (there can be caching DNS servers that ignore the TTL of records), so you should always give it at least 24h by serving the same zone from both (if you've changed NS records). I'd instead avoid that worry by using non-registrar provided DNS servers.
I would hope that TLD registries that fail to provide this are very few if not non-existent.
[0]: https://www.sidn.nl/en/about-sidn/what-we-stand-for