Would it be possible to create your own phone and have it be fully using libre Hard&Software?

I see. Which device do you reccomend then?

Anything Replicant supports.

Note that even if you CAN make a libre baseband, it'd still be just as good an idea to have physical disconnects for wireless com chips, because why wouldn't you?

This is how the Librem 5 plans to get around the problem of needing a legally certified baseband chip, plus ensuring the baseband chip never sees unencrypted content. Since your messages have to go thru an untrusted server at some point, just moving all of your precautions upstream of the baseband will address at least some of the issues.

But would it be able to change it's own IMEI as simply as chnaging MAC on wireless cards? As far as i know, Mediatek chipsets can change their IMEI pretty easily.
Will it be possible to completely remove the baseband chip and replace it with other if changing IMEI is impossible/illegal?