what should replace my airport time capsule

Apple was the only supplier that ever offered a product like the Time Capsule, which was a router with a built in hard drive that was designed to store backups of a Mac using Time Machine.

So, a pure replacement, or "all in one" product is not going to be possible. The closest that you can come would be a router from either Synology or Asus that will support Time Machine backups to a separate USB hard drive connected to the product. So, instead of a one-piece solution like the Time Capsule, you will have a two-piece solution......a separate router and separate USB drive.

https://www.asus.com/support/FAQ/1011283/ https://www.synology.com/en-us/products/RT2600ac

Another option would be to purchase a router of your choice, or maybe one that your Internet Service Provider (ISP) provides and then add a separate Network Attached Storage device (NAS). The Synology NAS products are very popular for this type of use.

https://www.synology.com/en-us/support/nas_selector

I would get

Mesh Wi-Fi Networking - All Accessories - Apple

(I already did)

and then an external hd connected by usb-3 or higher or thunderbolt for time machine that connecs directly to the mac, one can also get NAS which support time machine