[The_Prodigy1982]

In the previous macOS High Sierra I think... it was easy to add ringtones to iTunes and sync them with your iPhone.
But now in Catalina, how should I do this?
(there's no Tones tab within Music aka the former iTunes)

[doogald]

According to https://support.apple.com/en-us/HT207955 :

If you created a custom ringtone in an app, like Garageband, you can sync it to your iPhone, iPad, or iPod touch using iTunes for Windows or the Finder on a Mac.

1. Connect your iPhone, iPad, or iPod touch to your computer.
2. On a PC, open iTunes for Windows. On a Mac, open the Finder.
3. Drag and drop the ringtone to your device.

So, it should be the same way: just drag-and-drop the actual ringtone files to the iPhone device in the sidebar.

[The_Prodigy1982]

According to https://support.apple.com/en-us/HT207955 :
So, it should be the same way: just drag-and-drop the actual ringtone files to the iPhone device in the sidebar.

Well, I forgot to mention it in the opening post... but I DID try that, and it doesn't work at all.
The new ring tone doesn't show up on my iPhone.

[JML5150]

At this point you will probably have to use a 3rd party program like iMazing or Dr. Fone for custom ringtones. I personally use iMazing.

[Just_Me_D]

In the previous macOS High Sierra I think... it was easy to add ringtones to iTunes and sync them with your iPhone.
But now in Catalina, how should I do this?
(there's no Tones tab within Music aka the former iTunes)

On your Mac running macOS Catalina, connect your iPhone and then initiate the iTunes app. Yeah, I know that the iPhone is managed via Finder nowadays, but trust me.

After starting the iTunes app on your Mac, select your iPhone on the left side of the screen. Drag the ringtone (.m4r) and drop it in the open space in the middle of the screen.

On your iPhone, go to Settings → Sounds & Haptics to select the section you want to associate the ringtone with. Afterwards, locate the newly installed ringtone among the listing therein.

[The_Prodigy1982]

At this point you will probably have to use a 3rd party program like iMazing or Dr. Fone for custom ringtones. I personally use iMazing.

iMazing was VERY VERY easy to do the job, thanks!
Hopefully Apple will soon fix this issue again... but for the time being iMazing wil do.

[Just_Me_D]

iMazing was VERY VERY easy to do the job, thanks!
Hopefully Apple will soon fix this issue again... but for the time being iMazing wil do.

Dragging and dropping the ringtones using iTunes on macOS Catalina was quick and easy for me.