I've found an open-source local note-taking app Quillnote and I think it is helpful if you want to manage your quick notes like Google Keep and expecting Markdown supported. Everything you need is just "share" your quick notes between Quillnote and Joplin. Nothing more, just to provide a solution.
Edit:
As @kai said, Quillnote GitHub repo is not under maintaining anymore but fortunately there is someone's cloned version named Quillpad is now in active of development.
Thanks again @kai!
Thanks for thre tip; hadn't seen this before & it's really a very nice app. Since I also suffer from the "Joplin takes ca. 30 secs to launch under Android" problem, jotting something down quickly in Quillnote and then sharing it with Joplin is a great workaround.
Yes, it's great for mobile to mobile but better to stay single direction sharing quick notes from Quillnotes to Joplin, it can be avoided most syncing problems on the desktop side.
If you work frequently between mobile and desktop, make sure you've opened the Joplin app on mobile and wait until the data sync from mobile to desktop each time before working on the Joplin desktop.
Footnote: looking at their Github entry it appears that the original Quillnote has been discontinued, but there is a fork unter Quillpad continuing the work; also available on F-Droid.
Cheers,
kai.
Oh! Thanks so much for reminding me about it, @kai ! That's great! The Quillpad repository appears to be active.
Even if you wait for the message declaring that x items have been created, deleted, updated you can run into synchronisation conflicts in my experience (haven't quite found out the conditions to reproduce them).
Is it advisable even to press the sync button manually before you leave the mobile app? Shouldn't be necessary though as the "fetched" message refers to the client, not the server. And pressing on sync starts a procedure which can take as long as the initial sync when starting the mobile app 
In my use case, I only share the quick note which is already completed, and just share it with Joplin for backup. So, each one of my quick notes will only share once, and after that, I will delete that note from Quillnote.
(p.s. I didn't try Quillpad yet, so I don't know if they have something different).
One will be created one new note to Joplin each time you share from Quillnote, so it's not possible for updates the same note file. It wouldn't create any problem because the "share" behavior is based on plain text. Just don't expect to overwrite the same note file again and again due to the encryption syncing process of Joplin will turn your notes into a different format.
Overall, while it would be duplicated notes in some situations such as I share the same note twice, I didn't experience any conflict issue in the above use case.
That's just how I do it with the Drafts app on iOS. If sharing starts an initial sync, however, quick notes become slooow notes as it can take anything from 30 to 120 seconds (OneDrive as target).
Subsequent sync actions are considerably faster, but as @Kai pointed out in this thread launching Joplin mobile is simply no fun...
Umm... I think you can create a new profile to only contain your quick notes. Or separate archived notes to a new profile? Or grouping notes that are unnecessary to be encrypted? It should be the only way to increase the sync speed when using whatever dropbox and onedrive... 
Never mind the speed - public cloud access in combination with E2EE definitely belongs to Joplin's unique selling points.
I'd be interested to hear how subscribing to Joplin Cloud or setting up your own server influences the speed of Joplin for Android or iOS specifically (i.e. the number of seconds it takes to launch and perform initial sync for the specific session).
You should give the Joplin cloud a try for 14 days and see for yourself, and it won't disappoint you in the performance of syncing. 