update Catalina

You will have to decide for yourself if it "is wise." The following links should help you decide if there are any problems:

1. How to upgrade to macOS Catalina - Apple Support
2. Catalina - an upgrade guide - Apple Community
3. How to prepare the Photo Libraries before… - Apple Community

Now that 10.15.6 solved the Mail issue, I would say ok to upgrade. (Always have a current backup before major upgrades.)

Keep in mind that 32 bit Apps are no longer being supported though— however, enough time has gone by, I've found most developers have 64 bit updates or have told their customers about their plans therein.

I wholeheartedly agree with Ronasara. You should check to see which of your current apps are 32 bit and won't work with Catalina. You can use the free GO64 app to see which of your current apps will have to be upgraded or an alternate found.

Also before upgrading make sure you have a full and current backup of your boot drive in case you'd like to go back to Mojave.

In addition to the information you were given, and that you should look at carefully:

what is your mac model?

how much RAM?

how much free disk space?

Check if all the applications you rely on are compatible or can be updated to a version compatible with Catalina.

And, most of all, NEVER do such an upgrade without having a complete backup.

All suggestions made are excellent. To add some additional thoughts: Catalina does seem to use more RAM, so computers which are deficient there may encounter slowness. Specifically any older apps which are still 32 bit versions will no longer function. They will have to be updated, if possible, or upgraded at a cost to you. I would check this carefully to ensure that you don't incur an unexpected expense.