They only slow down player market activity for those who can’t wrap their head around how the market works. It is possible to make hundreds of billions ISK in EVE a month. Just doing trading. Those who know how to do it don’t have to have any supplemental income at all. You only need supplemental income in the beginning, when you don’t yet have sufficient funds to diversify your investments and can’t absorb losses.
Taxes are fine where they are. Market order slots can be maxed to 305, I think is the max number, with skills. If you need more than that, you’re gonna have to train a new character. That might be inconvenient, and it’s possible CCP is open to increasing the number in the future if they see a legitimate need for it.