I know GPA and SAT/ACT scores will matter a lot of transfers, but do colleges look at HS Extra Curricular activities as well, just like acceptance right after high school?
If you will be below junior level when you transfer, then expect high school things to matter significantly. How much each thing matters depends on the college you apply to as a transfer.
It’s going to vary from school to school, but after only a single semester of college your high school record is still going to carry a lot of weight. HS ECs might not be the thing to make or break an application, but in a situation such as yours they will definitely be looked at.
if you have good ones, I always say put them in. No harm, no foul. Freshman and sophomore years not important.