How to recover my iCloud account without linked phone number

Nobody here works for Apple so there is nothing we can actually "do" to help you with this other than provide information.

If you used your iCloud account as your notification address that was a big mistake. You can't get the emails that you need to see when you have an issue with that very account. Once you get this sorted out use a different email account.

You talk about security questions. Apple stopped using those several years ago and has been constantly reminding people to change to the new security method. Did you really mean security questions? If you use them and they still work then fine. If they don't and can't find a way that Apple will accept that you really are the account owner then you may have to abandon the account.

A thief who steals your phone while you are using it could possibly get the IMEI or serial number off it. That's not security. In fact with older phones I seem to remember they even printed IMEI and serial numbers on the outside of the phone. Anyway, if you owned a secondhand phone and the old owner kept those numbers and had some basic information about you, having those numbers as security wouldn't be secure.

Have you been using account recovery? How to use account recovery when you can’t reset your Apple ID password - Apple Support

Apple says you can use card information, but they may require more than that. Realize that accounts these days can have huge financial and personal information on them. Accounts used to be trivial. Now they are something big and entail security.