You should however be able to use the "Open…" command in Textastic to open individual files from Synology Drive.

For this to work on iOS 13, the file provider, in this case the Synology app, needs to add support for opening folders. 

I know that this currently works with iCloud Drive, "On my iPad/iPhone", Working Copy (Git client), and Secure ShellFish (SFTP client). 

File providers that do not yet support this are greyed out when trying to use the "Add External Folder…" command in Textastic.

iCloud pretty much does its magic in the background. For more control you should consider using other methods like the built-in SFTP client. Using it, you can manually download and upload files. See https://www.textasticapp.com/v8/manual/remote_servers/sftp_ftps_ftp.html

Since you seem to be familiar with Git, also have a look at the Git client app Working Copy - Textastic can open Git repositories from Working Copy as external folders: https://www.textasticapp.com/v8/manual/integration_other_apps/git_client_working_copy.html

Files you edit are downloaded to your device so you can work on them offline. iCloud periodically uploads your changes when it has an internet connection and it does automatically sync when you go back online.

I'm glad that you found out how to select a different syntax definition. Sorry that it wasn't more obvious.

With iOS 13 Apple changed the way external folders work. File provider apps need to be updated to support folder picking. So currently, this works with iCloud Drive, "On my iPad/iPhone", Working Copy (Git) and Secure ShellFish (SFTP).

I recommend to use Working Copy for Git.

Since this is implemented as an accessibility feature on the OS level I don't think apps can influence what happens when using the mouse on iOS 13 beyond what Apple already offers as configuration options.

Thanks, I'll look into it.

Just syntax highlighting, it does not run code. 

I'll consider it for a future update. Go to line is already implemented by the way.