Photo Storage, External Hard Drive, Dropbox, Need Space

Hi, I am running Photos for Mac and my computer has 120 GB. I have a lot of photos and I also TAKE a lot of photos, and I have run out of space. So, I'm trying to find a solution going forward. I'll write a brief list of what I've tried so far and the services I've subscribed to, but I'm hoping someone can help me create a system that will work in the long term, especially so that my photos and videos are safe and I'll still have multiple backups.

I have dropbox and automatic camera upload, and I also have Google photos which backs up the non high-res version of everything. I bought a big external hard drive today and copied my Photos library over to it, but now I'm thinking...if I plug my iPhone into my computer and it starts to automatically upload all my photos BUT my external HD isn't plugged in...where are those photos going? A different photo library? How can I get the recent uploads to the hard drive?

Is dropbox a solution? Do I just copy the entire photos library into dropbox? If so, do new uploads into photos automatically head to that dropbox folder? Would I have to keep that dropbox folder synced to my computer (in which case...there isn't enough room on the hard drive...).

Additionally, the photos library on this mac is 35 gb, but this mac is relatively new. My iPhotos library from before has 250 gbs and I'd like a way to combine all of those old photos into one solid library, but with only 120 gbs on this computer, how can I manage to consolidate it all?

Lastly, going forward I plan to start using an SLR and editing photos...what kind of storage solution/system am I going to need? I'm thinking that lugging around external hard drives with photo libraries on them might be clunky?

I hope these questions are clear, and I would appreciate any input.