They do slightly different things.
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Time Machine (1) keeps older versions of files. So if you want to go back to an older version, or if a virus corrupts your files, Time Machine has the old versions. (2) keeps a full backup of your filesystem including apps. So if an update goes wrong and you want to roll back to your old system setup, Time Machine can do that.
What Time Machine doesn't do: It doesn't provide an off-site backup in case your house burns down. It doesn't let you share or sync files between computers and devices.
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What iCloud does: (1) Provides an off-site backup of documents and photos for disaster recovery. (2) Keeps your photos and files synced between your machines, phones and devices. (3) Lets you save disk space on machines with smaller storage by offloading old / unused files and photos.
What iCloud doesn't do: It doesn't keep old versions of your files. It doesn't back up apps. It doesn't let you roll back bad updates. If you have 'save HDD space' on and a lot of files, iCloud might contain the only copy of a lot of your files / photos, which carries its own risks.
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FYI you could attach your current TM HDD to the Mac Mini and have both the MBP and the Mini back up to the same HDD. (ie. leaving it permanently attached to the Mini.) The first backup is the slowest and best done directly attached, (you already have this for the MBP) but subsequent backups are much smaller. The MPB can backup over wifi to the TM HDD on the Mini.
For what it's worth, I no longer run Time Machine. All my machines back up to a 2TB iCloud storage space. Anything goes wrong, I can easily re-download any apps from the internet, and with the Mac App Store, restoring software installation and re-entering licensing is no longer the headache it used to be.
However I'm uncomfortable with the sole version of a lot of my files/photos only existing in iCloud, so I will soon buy a cheap used intel Mac Mini just to keep a full local copy of my iCloud 2TB space.
