Side effects of Android CarrierConfig reporting a foreign network as ”not roaming”
I can not add a comment in your question because I have not the neccesary score to do it. So, the only way is doing a new answer 
As I explain you in the other question you could add to any network, any other network of other country for that it do not behave like a roaming.
I am going to explain my point of view about the problem to develop this feature. It is true that although roaming in europe is free depends on some factors as you said, for example:

As of last year, EU carriers can no longer charge roaming fees for use on other EU networks, as long as the majority of usage is in the “home state” of the SIM card, or the subscriber can demonstrate some ties with that state (e.g. residence, work, study or family).

If LineageOs develop this feature and for example you have a French SIM and you live in Italy, there will come a time when the French operator will start charging a plus for the roaming.
And it could be a problem, because if the user have not activated data roaming, the user won't know that the operator could do that.
The user need to know that it is possible that the roaming may produce an extra cost on your bill.
It is possible that you has a French SIM because you are french, and you live in Italy because your work is in Italy, and you could think, &quot;Ok, the European single market should allow me to use the same SIM in different countries at no cost&quot; and that was the original idea of the end of roaming.
But some operators (the big carriers) complained that people could buy SIMs in countries with cheaper rates to use them in countries with higher rates.
So, summarizing, I think that it is possible the feature that you want (technically talking) but it is not recommended.