Nope. Just using the document provider option. It means DT is in control of the files, you just read and write them.
https://www.textasticapp.com/v9/manual/integration_other_apps/external_files_folders.html
I use the “add external folder” option in secure shellfish and working copy frequently. I’ve only really needed to edit the odd file in DT on iOS (if it’s something I’ll edit, it lives in Obsidian for me).
Sorry for being thick, but I just don’t follow. DT syncs my DBs with DTTG. I don’t have a DT folder in my iCloud that I can use as an external source with Textastic. What am I missing here?
While I also sync DT3 towards DTTG via iCloud (CloudKit), I can enter my DTTG Database for example from LiquidText without any problem. So there must be a way to do so, even without a dedicated folder in the iCloud.
+1. DTTG and DT3 have useless editors. Can’t really use their markdown or rich text editors to take long notes, 1990 editors on 2022 machines. Hassle having to resort of iA Writer or Drafts on both Mac and iPad to write then send to DTTG/DT3. Worse is when writing rich text notes externally which DTTG/DT3 screws up formats once imported. DEVONtech should vastly improve their editors. Slowly I’m moving towards Obsidian. Craft looks good too.
Has using an external editor become difficult with DEVONthink? I don’t remember it being tough to do. And isn’t Drafts’ specialty sending text to other apps? And doesn’t DT and DTTG sync with each other? Why mention “send to DTTG/DT3” as if it were a manual process to send to both.
I’m very confused by the description of your workflow. 
agree devonthink editors are rubbish + they are too proud to change
Never said it’s tough to send to DT3/DTTG, I said “hassle” as need to open iA Writer/Drafts and remembering to send to DT3 end of the day. Typical days are filled with running meetings with my team and visiting customers. Crucial to take good notes and do them fast, my memory is not that good, plus OCD with alignment and proper formatting 
I’m using DT3/DTTG pro-Drafts Pro-iA Writer on Mac, and same apps on iPad when meeting customers. DT/DTTG are good for storing and sorting but really horrible for note taking. Alternating between iA and Drafts, iA Writer is nicer, distraction free and can be launched via external app on Mac DT but Drafts have actions and workspaces. Taking rich text notes is even worse on DT/DTTG.
It used to be more complicated, but now all you should have to do is:
Open DTTG and find your markdown file.
Touch Share > iA Writer to open the file in iA Writer
Write in the file in iA Writer.
Switch back to DTTG, and see your changes.
That’s basically the same process as the Mac - no need to send the file back from iA Writer. Once you’ve done step two, if doesn’t need to be repeated in that session (as far as I can tell).
Apologies if I’m missing something!
Opening files in-place should also work with most apps. Try using Open in Textastic and then opening files from DEVONthink databases provided that you have enabled DEVONthink in sidebar in the Files app.
(For this to work, security in DTTG should not be set to lock databases “Immediately”.)
@brookter and @iApple may also try this instead of sharing.
This works fine with most apps but there are issues with saving back changes to DTTG with some of the apps (most notably, PDF Expert).