If you choose to report your stack gains/losses as a day-trader (i.e. Schedule C, allowing you to deduct various expenses), it doesn't make any difference whether you are doing full-time or part-time.
"Day-trader" status is your choice - you don't have to subject yourself to self-employment tax.
If you want to report your stock gains and losses on Form 8949 and Schedule D, you can do that for 2020 (Investment Income interview). You can amend prior-year returns, but may find it difficult to recover any prior-year self employment taxes - that money goes to the Social Security Administration, not the IRS.
