USING APPLE PHOTOS WITH EXTERNAL DRIVE MAC
So you can leave icloud photos on on the phone, export to the mac, then delete from the phone to clear space in icloud and keep them on your mac in the Photos app. So one device has to have iCloud photos disabled, in this case, your mac. There really is no way to use iCloud photos and keep _all_ your photos while deleting them from any device using iCloud photos. If you leave iCloud on, then you'll gradually use more and more storage as you take new photos so you'll have to remember to import those photos to your mac and delete them from your phone regularly. Once you're all configured, you can purge photos from your phone, disable iCloud and be on your merry way. Test this by deleting one photo from your phone - if it vanishes from your mac's photo library, you still have iCloud on on the mac.
Whatever you do, don't delete anything from your phone until you're 100% sure your photos are saved locally (not just thumbnails, but actual photos) and iCloud is OFF on your mac. iCloud will then have nothing to do with your photos. To add new photos, just import them from your phone on a regular basis. You can then use the Photos app and manage your library locally. All your icloud photos will be downloaded into your photos library on the external drive. Then turn off iCloud and choose the option to store a local copy of all your photos. Physically MOVE your photo library from the mac internal drive to the external drive (very easy).export all your photos out of the photos app into the external drive and they will then just be individual photo files.If you turn on icloud photos on your mac, all your photos there will mirror your iphone's photo camera roll, etc. To answer your other questions, yes, you can use an external drives to store your photos that now reside in iCloud.
I'd also buy two and not rely on a single drive to store my data, especially photos and work/school documents since any drive can fail. Tremendously faster and more durable as an external device than spinning disks. I would go for an external SSD and not a traditional HDD if you can afford it.