Apple stopped selling Time Capsules nearly four years ago, and even the most recent are now at increasing risk of hardware failure, particularly their hard drives, which are now becoming significantly more likely to lose all of your backups.
There’s no direct replacement which works as well as the original, and in any case you should take the opportunity to reassess how you make backups. As you’re backing up desktop Macs over a wired connection, your first choice should be to attach local storage to each rather than going over a network, as that’s much faster and more reliable, particularly now that Time Machine can back up to APFS. Although that works best to an SSD, even a USB–C hard drive would bring great benefits over your Time Capsule.