I survived for a long time with a free Dropbox account using my Mac and iPhone. Scrivener projects are not huge — unless one adds in humungous JPEGs or other non-textual files — you would be hard pressed to exceed the limits of a free account.
Eventually I took out a paid Dropbox account not because the space limit was reached but rather I added more devices than a free account would allow.
Aside from Obsidian, may I ask what tools you have generally decided on?
Well now, that could be the proverbial “game changer.” I just tested and export of my book in markdown format and imported to both DT and an Obsidian vault. That works perfectly. To avoid going over the free tier of Dropbox I can archive completed material. Of course, this means they are not longer formated in Scrivenr but I’d still have the backups that I can restore.
Am I thinking about this correctly?
The zipped backups ARE in Scrivener format and can be restored using a Scrivener menu option. Yes, you could archive completed material but those backup are essentially providing that already.
Thank you for your kind help! When you are finished with a Scrivener project and want to free up Dropbox space, what do you do with the project file; do you just move it out of Dropbox and to your Mac and or external drive should you ever need it?
When I finish a project I just leave the Scrivener files on Dropbox. Right now they occupy less than 200Mb — a small part of the 50Gb of my material on Dropbox — and with my paid-for 2Tb account meaning there is no requirement for me to move them off. And all the files are marked as offline in Dropbox.
Thank you. I do appreciate the kind help. 
Right now it’s more of a what tools have I decided I no longer need 
DEVONThink
Omnifocus
Scrivener
Aeon Timeline
Scapple
1Password - replaced by Strongbox
Closing on on removing these apps but not totally ready to turn them off yet:
BookPedia
CDPedia
DVDPedia
ChronoSync
VueScan
EpsonScan2
And then there are those who role is much reduced
Contacts (only current ones still in there, Farley file is in Obsidian now)
Goodnotes - Obsidian and Shortcuts with Actions for Obsidian and Readwise Reader has replaced much of what I used to use Goodnotes for
LibreOffice
Highlights
PDF Expert
And the apps who have also gotten a lot more attention now
Text Edit
Android Studio
Pycharm
GitKraken
DB Browser for SQLite
Reader by Readwise
Keyboard Maestro
Zoom
Thinderbird
Google Earth Pro
Shortcuts w/ Actions for Obsidian
And the ones that are at about the same level of use
Zotero
Banktivity
NameChanger
Fetch
Carbon Copy Cloner
Hazel
CleanMyMac
SpamSieve
TimeTable3
LightRoom Classic
Photoshop
iMovie
SlideSnap Pro AutoCropper
Batch Crop
A Better Finder Rename 12
Calibre
PhotoSync
iMazing
Slack
Google Chrome
Hope that helps
That is quite the list! 