Selfhosted

So I've been working through this myself recently. I was pretty deep into the ecosystem and frankly there's not a replacement for everything, but most of it's close enough for me. Much of this will depend on what you're really wanting to accomplish.

For Mac you didn't mention whether it's an Apple Silicon or Intel device, so this is going to all be specific to my experience which is with Asahi Fedora Remix on an M1 MacBook:
I use the GNOME version since I think the UX has some Apple familiarity and works well for laptops but Plasma is probably a better choice for a desktop.
It's generally very stable but it doesn't really have anything yet to easily translate x64 apps to ARM - there's muvm, but if what you're trying to run isn't covered by the included libraries (e.g. Electron apps) you're going to have to build an overlay filesystem image containing the dependencies you need; it doesn't "just work" like Rosetta (or in the Windows world, Prism).
You will also lose some of the stuff like Touch ID because reverse engineering the SEP and sepOS are still in progress. Battery life also isn't as good.
Lastly, Linux desktop security simply isn't comparable to any modern build of macOS. TCC on macOS provides incredibly robust protections for user data and device access that Windows and Linux simply don't offer (yet). On the bright side with Asahi, you do still get some decent protection against low-level compromise because each OS ("container") on Apple Silicon has its own verified boot chain and recoveryOS, which is largely immutable, can in theory be used to nuke an infected install.

I replaced my iPhone with a Graphene Pixel, but I mostly just use it for web browsing and email and I was an Android user for a long time prior so I can't speak to whether this would be an easy transition or not.

My replacements for Apple services so far are:

• iCloud Drive: Nextcloud with a Managed Service Provider with Cryptomator to approximate the end-to-end encryption offered by the Advanced Data Protection option.
• iCloud Private Relay: Mullvad, though any trustworthy VPN should do. Note that it's not an exact replacement because iCloud Private Relay traffic originates from Apple IP ranges which tend to have a much better reputation than VPNs and thus run into anti-bot measures less frequently.
• iCloud Mail, Calendar, Contacts: Tuta mail + Addy.io for the "Hide my Email" aliasing component.
• iCloud Keychain: Proton Pass, though whether you'd want to trust Proton at this point is another topic of discussion that's out of scope here.
• iCloud Photos: I've recently tried out Zeitkapsl and have been extremely pleased with it. Their desktop app originally didn't have an ARM Linux build so I reached out to see if one might eventually happen and within a day or two their team had one ready.
• iCloud Notes: This one is tough to replicate. It's so simple, yet it's probably one of the most valuable products for me in the ENTIRE Apple ecosystem and one of the few utilities I actually found value in having synced between all my devices. I've been using Obsidian with the .md files synced in a Cryptomator vault to replace it, though I may end up subscribing to their sync service just for ease-of-use.

There are other viable options for most or all of these; I had a pretty specific set of requirements in mind when making my choices: no documented data collection or sharing beyond what's absolutely strictly necessary, no Google analytics or push notifications, European owned and hosted, end-to-end encrypted where possible, no advertising, no profiling, etc.

If you have any questions let me know, hope this helps though!