@cantthinkofone - You cannot set the default properties of connection lines.
There are a few Wish List Idea posts that you could include your asks in as comments (and upvote the post):
However, if you feel that idea doesn’t truly reflect what you are looking to accomplish, you can always add your own Idea in the Wish List category by following the guidelines here: Wish List: Everything You Need to Know.
If you do create a Wish List Idea post, I would also recommend that you post a link to it back here, so that future readers of this post can quickly get to and vote for your Idea.
Lastly, don't forget to vote for your own Idea.
FYI: The only shortcuts I know related to connection lines are:
To draw a straight horizontal, vertical, or diagonal line, start drawing a line and hold Shift.
To disable snapping to other objects hold Ctrl (for Windows) or Cmd (for Mac).
Drawing a straight line and a standalone line
More on Connection Lines in the following Help Center article:
https://help.miro.com/hc/en-us/articles/360017730733-Connection-Lines