FWIW there’s a script on this forum for pasting from Bike as an outline into macOS Mail, using Mail’s native indent format.
Not quite clear here whether you are talking about
Bike -> Mail
orBike -> Notes -> Mail
?
If the latter, then perhaps this is essentially a problem with copying from Notes to Mail, rather than a difficulty in exporting outlines from Bike ?
Perhaps keep the outlines in Bike ? (In Notes, just paste links to the Bike outlines ?
Bike > Edit > Copy > Row Link
, orBike > Edit > Copy > Path Row Link