Can I use Time Capsule only as iMac backup drive without using it’s Router? If yes how do I make that work? Thank you.

Yes. Connect the TC via Ethernet directly to your router or through a switch, perform a "hard reset", and then use AirPort Utility to configure it.

How to reset your AirPort base station - Apple Support

AirPort Utility User Guide for Mac - Apple Support

The "hard reset" has no effect on the TC's existing backups which will remain untouched, but you will need to "reconnect" to the TC by removing it from Time Machine, and subsequently adding it again at its new network location. Time Machine will let you know that the TC already has backups of that Mac and ask if you want to continue using those backups or create a new set. Choose the former. TM will pick up where it left off.

You can, however IMHO that is not a wise move as your TC is now pretty old which means the HD inside of that is used from Time Machine is also very old. What I and many other experienced users on these forums like is the OWC Mercury Elite Pro due to its reasonable cost, high quality and the service provided by OWC.

👍

That's what I've done with every Mac since Time Machine's beginning.

And, none of mine have ever failed, and I have every model ever built. Of course anything can fail, and will eventually, but the TC's internal hard disk drive can be replaced — with difficulty — if necessary.

You can also connect any number of external (cheap, disposable) HDDs to the TC's USB port.

this is exactly what I have done, even using the time capsule to migrate from old iMac to new M4 iMac.

Hi,

I replaced Time Capsule HDD with 2TB SSD which is very comfortable and makes it more safe back up system.