>As much as possible, bring just a small backpack or duffel that fits under the seat? Then you can also skip the check-in line and just use your phone for check-in and boarding passes.

What? You can bring a full-size carry-on suitcase AND a backpack (or personal item) without dealing with any check-in lines for checked bags. You don't need to downsize this minimally for carry-on benefits.

And if OP is flying frequently, they are going to get status in a month or two and carry-on access is going to be a non-issue.