TYPO3 Core

TL;DR:
Yes, indeed using TCAdefaults for FlexForm fields is currently not possible. (You would need to adjust the <default> attribute in the flexform for this.)

Setting a label is possible via:

TCEFORM.tt_content.pi_flexform.ghsvgcrop_svgcropdemo.sDEF.settings\.subtitle.label=Your custom label

Now the in-depth take on this.

TYPO3\CMS\Backend\Form\FormDataProvider\DatabaseRowInitializeNew::setDefaultsFromPageTsConfig() only interprets existing DB field names.
The check is done on $result.processedTca.columns.XX - so you could only access "pi_flexform" here, but not "pi_flexform.yourKey". Said
initializer doesn't know about flexforms yet.

That is performed in TYPO3\CMS\Backend\Form\FormDataProvider\TcaFlexProcess and you can actually see some documentation on its restrictions:

https://github.com/TYPO3/typo3/blob/main/typo3/sysext/backend/Classes/Form/FormDataProvider/TcaFlexProcess.php#L179

Now the problem within that Processor-Class is, that currently flexform operations don't really distinguish between fetching the current value from the database,
or from a flexform-inherited default (at least to my casual interpretation).

Generally, operating on the result set within that Processor is a bit painful.
From what I've heard, work will be done in this are in v13, so this might be addressed.

I took a very deep dive into the rabbit hole and tried to patch it just for fun. I partially succeeded (see https://review.typo3.org/c/Packages/TYPO3.CMS/+/81548),
which allows one to set:

TCAdefaults.tt_content.pi_flexform.settings\.subtitle=My override value

But sadly (due to the issues mentioned earlier) this override gets applied no matter if a record is fresh or read from the database, so that's not a helpful state.