and no, forwarding as attachment, rather than inline, won't help here, because emails as attachment are now mostly rejected by the likes of gmail as they're virus/malware vectors.

Well, actually you do have that feature. When you set to compose in HTML and use the compose default of variable-width font and medium size, TB always sends plain text UNLESS you stylize (e.g., bold, italics) the message, or use an HTML sig. I suggest to also set the tools>preferences>composition setting in 'send options' to tick the box to send plain text when possible. I have used this setting for over a decade and it works well.

However, leaving it to compose as HTML will still then have the whole bold/italic/bullet list/etc wysiwyg styling bar, which I'd never use. So, not quite.

If you set up Account settings to not use HTML and you create and send a plain text email then there is no formatting - no html - regardless of whether your send options say to use HTML. It cannot use what does not exist.

However, leaving it to compose as HTML will still then have the whole bold/italic/bullet list/etc wysiwyg styling bar, which I'd never use. So, not quite.

If Account Setting say to use HTML and your 'Send Options' say to send in both plain text and html. The Write window will display the 'Formatting Bar' whether you use it or not is up to you. But in these cases there is the basic HTML in the formatting such as the html head body and meta data. It also includes the body text colour and the background colour as per default or chosen settings. So even if it looks like plain text, it is not unless you choose to view the sent mail as plain text (View > Message Body as > Plain Text) because that option was also included in the send options.

In other words, any included formatting can be stripped to show as plain text, but you cannot add html formatting that never existed.

You do have the option to disable the display of the 'Formatting Bar'. In 'Write' window: View > Toolbars > Uncheck 'Formatting Bar'