How can I use keychain without 2 factor authentication?

Like, not pop up every five seconds?

Not expected behaviour.

Many come here complaining about 2FA and find there's an underlying issue unrelated to 2FA.

I go weeks without needing a code. YMMV of course, but every 5 seconds is a sign of something else.

I resisted using 2FA for years until I set up my iPad Air 3. When I turned on Keychain, which I really wanted to use, I relented. Once I got used to it, it wasn’t bad at all though it does seem that I have to trust my devices more often than I should have to. I have 4 trusted Apple devices that can receive the codes, my phone number and my wife’s phone number as a second trusted number that can all get codes.

You really shouldn’t be getting prompted to enter the codes all that often and it should be seamless for the most part. You have several Apple devices so once you make the devices, trusted devices, you can get the code on the device that is asking for the code. If you choose to set it up, make sure that you also have one additional trusted phone number.

It may be annoying to you, but there is no way to use Keychain in iOS 13 without enabling two factor authentication. You cannot get around it.