Deliverability: Sending emails from a personal email - when it's a newly created email

Don't worry about general reputation at the local-part level. (The local-part of an email address is the left-hand-side, i.e. first.last in the address first.last@example.com).

Reputation is ultimately based on the shortest private domain, not the local-part or even subdomains. That is, in first.last@mktg.example.com  it's example.com that needs to be clean/warmed/trustworthy.

Also, unless you have branded envelope sender set up in your instance, the true SMTP sender of your Marketo emails is <unique_verp_part>@<something>.mktomail.com, not first.last@example.com. (The sender that appears in the From: header and the SMTP sender can be completely different). So when your reputation is based on the SMTP domain, example.com isn't even considered, let alone its local-parts.

However! The above doesn't account for individual recipient allow lists/whitelists, or something specifically tweaked by a recipient's mail admin. If someone has previously added the From: address sandy@example.com to a local list, then jane@example.com won't be covered.

In any case, this is really the nature of the beast. Reps change and people move on. If there's a ding here and there, there's no solution unless you never send from real addresses, only marketing-controlled addresses.