yes, you are too late for contributing to HSA or SOLO 401(k) for 2020. the latest date was 5/17/2021. however, if you filed a timely extension for your 1040 then you have until 10/15/2021 to contribute to a SOLO 401(k) but not your HSA.
you can only convert a traditional 401(k) (you're an employee) to a ROTH 401(k) if your plan allows it but it can be done at any time. you will owe income taxes on the amount converted.
