I'm planning also on ordering a frame on Aliexpress and therefore asked German customs via E-mail about the current regulations.
It seems that you need to have luck and find a seller that cares about EU regulations. For Germany the seller/courier need to put a certain code on the packaging to prevent taxes. There is a rule for private customers that excludes them from all the high taxes. You just need to pay VAT and a small customs fee.
I wrote to two sellers regarding this topic but haven't received an answer yet. Until this is not clear I won't by on AliExpress.
I keep my fingers crossed that maybe AliExpress can support you. I would definitely write an E-mail to them. I had a similar issue on Alibaba in 2023 and Alibaba supported me quiet well and the seller refunded me. They declared completely wrong and I had to pay way to high taxes and fees.

Hi,

I want to build my first gravel bike and I'm looking for a good frame to start around 250-300 USD.
The goal is not to build the perfect bike, instead I want tolearn how to build abike and want to improve my skills and
upgrade the bike from time to time and also test different components (chains, cassettes etc.)
Do you have any recommandation? Alloy frame would be prefered, but carbon is also okay.
Thanks for the support. 

Thanks for your help.
I will check Seaboard GR-02 and Koozer GX5300