Changing the base currency of your account will not help you save money when the dollar/euro exchange rate changes. If you really think the USD is going to weaken further then you can sell USD in the CFD account and buy them back cheaper in the future. But this is a risky gamble. The USD/EUR rate is where it is because roughly equal numbers of people expect the USD to strengthen as think it will weaken further.
I would advise keeping your account in the currency where you live. This make keeping records easier, figuring tax liability will be easier, and it will have no effect on your investment returns. One of the great things about Trading 212 is that there is no fee on currency exchange and the buy/sell spread when you convert between euros and USD is tiny. So if you have a euro account and buy US stocks then that is fine. The only circumstance in which a USD account could be beneficial is if you were trading in and out of US stocks many dozens of times every day, in which case the buy/sell spread could add up to something significant - though it would still be very tiny compared to the buy/sell spread cost you would be incurring on the stocks themselves. If you are a day trader who is content to incur the spread cost on trading your shares, then the additional tiny cost of the currency exchange should not concern you.