Afaik, there is no separate default shape for the tempo envelope in the preferences. Also, this cannot be saved into default project settings.
However, you can select the current tempo envelope and from the right-click menu go to Envelope defaults/Default point shape. Set that to linear and all points created after that will be linear for the tempo envelope, which you probably knew already. This setting seems to keep as long as you have current Reaper session open. This is also saved with the project so will be there next time you open the same project.
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