The same humans who incorrectly add "www." when not told to are also unlikely to add "https://". So have the www. version redirect to the correct URL with HTTPS.
For that matter, since certificates with Let's Encrypt support arbitrarily many SubjectAltName (SAN) values, you can include the www variant in the certificate, so that your redirect can use HTTPS and HSTS.
The same humans who incorrectly add "www." when not told to are also unlikely to add "https://". So have the www. version redirect to the correct URL with HTTPS.
For that matter, since certificates with Let's Encrypt support arbitrarily many SubjectAltName (SAN) values, you can include the www variant in the certificate, so that your redirect can use HTTPS and HSTS.