As a photographer I already have my own photo management workflows: I use Lightroom to manage all of my photos and videos. The better ones are backed up to my SmugMug site in the Cloud, and the bulk of them (although I try to ruthlessly delete the bad and the mediocre) are backed up to external drives that I rotate so one is always stored at my office. Lightroom is overkill for most, and now that Adobe is really forcing the subscription model I would think hard before relying on it.
I use Google Photos to automatically sync photos and videos from my iPhone to the Cloud, and then back down to my Windows computer. I also use iCloud photo Library to sync between my iPhone, iPad, and the Cloud. You can never have too many backups. Automation is nice as well.
I have never splurged on the larger capacity iPhones, so I used to occasionally mass delete old photos and videos from my iPhone, but only after I had verified that they had been transferred to the Cloud and to my computer. The "Optimize iPhone Storage" setting has helped so I don't bother to delete. I also like the fact that Apple finally gave us a way to easily browse and save photos and videos from iMessage.
I will soon upgrade to the 8+ or the X, and I'm sure the improved camera functionality will inspire me to take many more photos with the phone and carry my camera bit less often. I might splurge on the 256GB model for the first time. I guess the only concern is whether or not having so many thousands of photos will impact the performance of the device by giving it more photos and videos to index. My iPhone currently has about 2000 photos and 300 videos, but the numbers would be much higher if I hadn't occasionally cleaned things out. I imagine the better camera performance in the newer models will put me up into the 5000 range quickly.
When it comes to dealing with thousands of photos and videos, I'm a big believer in redundancy and syncing. I have my external backups, my Smugmug backs (which is a paid service). My iOS photos and videos have additional backups to iCloud and Google. That covers the backups, but keeping things organized is just as important. You don't want to lose track of your most valued photos as videos. iCloud and Google both make pretty good attempts at automatically organizing things by metadata (date, time, place, etc.) and by image content (including automatically recognizing faces).
I'm the kind of person who has manually tagged images using Lightroom and other tools for years. The tags most of us care about are the ones that tell us who is in the photo so we can later find all the photos that have "Jane" in them, or perhaps "Jane AND Alex." Google and Apple are both pretty darned good at identifying faces, so maybe it comes down to what you prefer to use. If you are an Apple and macOS user through and through, I'd start with their tools for this sort of thing.
Sean