How do I turn off multitasking on Mac mini

First of all, happy birthday!

I am not familiar with Moneydance but from what you describe it seems it just has a busy and distracting user interface.

Some things need to show a lot of information. The way I cope with it is to use a large monitor, so I hope you have a nice big display to go with that Mini. You will need to poke around the Moneydance interface to see if there is a way for it to show its abundance of information in a more pleasing manner.

If you have the funds for it, an additional monitor might be worthwhile investigating. The new Mini will easily support two high resolution monitors. That way, you can spread out the information among (at least) two displays: Extend or mirror your Mac desktop across multiple displays - Apple Support

Multitasking is inseparable from macOS. You cannot just turn it off, any more than you could with Microsoft Windows or Linux installations. I am excluding UNIX single-user mode from this reply as that is not userland.

Multitasking as a computational concept has been incorporated in Mac operating systems since its initial implementation in System 5 (1987 or so).

I suspect there is more to your question. What is it you need to accomplish?

In Moneydance->Settings/Preferences under the General tab, do you have "Show All Accounts in Popup" checked? Also, do you have any internet connections set up to any of your financial services? That could be a source of popups.

FWIW, I use Moneydance and only ever get one window open.

You can’t turn off multitasking. The system uses it all of the time just to run components of macOS itself.

Multitasking just means the system can switch its attention between multiple things (more things than the processor has cores).

if you want to run a single application at a time (to reduce the amount of multitasking that would go on to support having several applications open at the same time), you are free to quit one application when launching another. However, unless your system is starved for RAM, there might not be much benefit in doing this.