moving from classic to cloud-based AND from PC to Mac

Yes, you can migrate a Catalog from versions of Lightroom  v6.14 (Perpetual) > v12.01 (Classic).

Consider that you may only have 20GB of Cloud storage space (Depending on the Plan you subscribe to). How much space do your photo files occupy locally?

Consider also that many Lightroom Classic features don’t exist in Lightroom-'Cloudy v6' . Some of the data gets transferred, and some information doesn’t transfer.
Data that doesn’t transfer to the Lightroom database includes:
• Folders and folder hierarchy (but you can convert to collections before migrating, which become albums in Lightroom).
• Develop History
• Snapshots
• Creations (books, slideshows, prints and web galleries).
• Smart collections.
• Color labels become keywords.
• Virtual copies become real copies.
• Collection sets become album folders.
• Collections become albums.

I am with @Theresa J  here- "you may want to stick with Classic.".!

With Lightroom (cloud) it doesn't matter if you use Windows or Mac. Everything is stored in a cloud. You simply sign in to your account on either computer and you will see your images.

However, migrating your content from Lightroom Classic to Lightroom could be a bigger challenge depending on how many images you have, and what features you depend on. Are you currently using Lightroom Classic? If so you may want to stick with Classic. I'm pretty sure you can open your Windows catalog with Lightroom Classic on a Mac. Hopefully someone who knows for sure will jump in here and confirm it.