Better Dropbox experiences via external drives

[Thomas P.12]

Dear Dropboxer,

the camera uploads of my 12 mobile devices of the whole family end up on my Drobbox, which of course I can't store on the SSD of my MacBook Pro due to lack of space, where I'm quite happy with the SmartSync feature.

However, when I'm editing movies with fcpX or iMovie, I also need offline access because some videos are a few GB in size. I synchronized everything on an external 5 TB disk; from there it's much easier to delete groups of photos and videos, as this function is unacceptably (!) cumbersome in the cloud itself via Dropox. As an old Dropboxer I remember that it used to be much easier (though not as easy as Google Photos there was an app to quickly select and deselect pictures, an app or add-on of which I forgot the name - Drobox should offer something like that to its customers again!).
Anyway, the fact that every time I want to work on editing my huge video project, I have to disconnect the Drobox link from my computer and set it up again for the external hard drive. This is annoying and error-prone.

I know, the Dropbox folder must be available at boot time! But why not allow for another hard drive based 100% sync-folder to be mounted and unmounted at any time without unwanted destruction of one's work? Please!
My USB disks are often not fast enough during the boot process to be recognized by the system. The Dropbox app starts before the drive is ready and produces the error message to reconnect to the Dropbox account or quit the program because no Dropbox folder was found.
On the other hand, if the external hard drive is disconnected from the computer while the Dropbox app is running, there is a chance that Dropbox will start deleting files it can no longer "see" before it realizes that all external memory has been removed.
I have been a victim of unnoticed data loss.

Is it asking too much to save space on my Mac with smart sync and still be able to use a fully synchronized external hard drive when needed?

Would be grateful for ideas. Currently I have not mounted the external hard drive. The start-up noise caused when the sleeping HDD when waked up by the finder is annoying. It appears even if I only use my SSD (loading or saving files that have no relation with dropbox).

Please, dear dropbox developers take this into account.

Your Thomas