Well, considering there isn't a single good third party equivalent to a Time Capsule, I'll go and buy one while they still exist. Sad day.
Another perspective:
"Alternatives to AirPort - Six Colors" (https://sixcolors.com/post/2018/04/alternatives-to-airport/)
What about the Time Capsule? When it launched, it was the definitive way to back up your Macs without plugging in an external hard drive. Very convenient, especially if your primary device is a MacBook! Here’s what I’d say: You’re better off backing up remotely to a cloud-based service like Backblaze, which is The Wirecutter’s pick. A Time Machine backup has the advantage of being fast and on your local network, but if something catastrophic happens to your home or office, you will lose your backup. Backing up to the cloud keeps your data regardless of natural disasters, and has other advantages including being able to retrieve any backed up file from anywhere.
That said, if you want a local backup server, your best option is to buy a NAS, short for network-attached storage. These are essentially giant hard drives with small embedded computers that act as servers when you put them on your local network. Wirecutter has a nice NAS roundup, and you’ll find that many NAS devices support Time Machine. Ideally, a NAS will also be able to back itself up to the cloud.