what would happen if a tx with min fee was approved by all the nodes, but not mined right away since the tx pool was really full, then a few blocks later the min fee increased a bunch so that old tx (still in the tx pool) had a too-low fee? would it get ejected from the tx pool?

Only if enough other txes are received with a fee/byte higher than this tx.

The default pool size is pretty large, though this is very unlikely.

Makes sense ty