What is the best Desktop Publishing software for a Mac?

Adobe InDesign replaced PageMaker as part of Adobe Creative but works much the same. However it is expensive and by subscription only.

www.adobe.comOfficial SiteAdobe Creative Cloud - Limitless Creativity for All

Affinity Publisher2 is in the Mac Apple Store and has pro featurs and good reviews at a more affordable one-time cost. Like InDesign and PageMaker, it has a steep learning curve but is quite good:

https://apps.apple.com/us/app/affinity-publisher-2/id1606941598?mt=12

Look into Affinity Publisher. Free trial for six months and you can then purchase it from the Mac App Store or directly from Affinity for under 100 USD. No subscription or Adobe BS. Available on iPad and Windows too.

Pages is not the right tool for serious DTP and you will find those serious qualities in Affinity Publisher.

There is also Pages, which probably isn't as advanced as the other two – but which comes with all Macs these days.

I forgot to add in my earlier post that the current version, Affinity Publisher 2, requires at least macOS 10.15 Catalina. If your iMac is not 2012 or newer, it cannot run Catalina.