On the surface the DNA tests seem to be useful but if you look deeper into it, you will see that they are absolutely unscientific. It is just that the companies behind them, that are interested in getting your DNA, are just very good at marketing.

In the US there is a very widespread trend of wanting to know your ancestry. Your modern history starts in the late 18th century, Europe seems to be so old and diverse, and I absolutely get the fascination. In fact, I am a hobby genealogist myself. I have researched almost 1,000 ancestors (not direct, also brothers and sisters). Some lines go as far as 1680, many are dead end around 1800, others are still waiting to be researched. For these results I have looked through thousands of church book pages, I visited smaller and bigger archives, I traveled through the country and paid money for census copies; as my approach is to have a primary source for each and every life event a person had.

Similar to your motivation it started out in an interest of where I come from. But I soon noticed that beyond my greats grandparents the people become really irrelevant and I feel no connection to them. For my identity, it doesn't really matter where these people came from. However, I use them to dig into the historical contexts they lived in. You get to know a lot about small villages and unimportant events in history you would have never heard of before. This is what actually fascinates me, similar to how some welsh myths or whatever fascinate you.

But let's get back to the DNA tests - why do I call them bullshit? Because they mostly are. It's not the molecular biology that is not far enough, it is the reference DNA these tests are using. It is possible to identity your ancestry on a continental level with these tests. In rare cases it might be even possible to say something like "eastern European", but most of the time this will be insignificant. Most importantly though, reading that you are 15% welsh is just bullshit. Even if we had the reference DNA of these populations - what are these populations even? What is an ethnicity? If you look at European history from the middle ages to the modern age, you will see that it is almost impossible to make out an "ethnicity". Just look at how the European population changed between 1618 and 1648 during the 30 year war, or how the Holy Roman Empire changed it borders throughout the centuries. In modern historic research, scientists are much more careful with the term "ethnicity", as the populations that are talked about were mostly made up in the 18th and 19the century during the development of national states in Europe.

I really, really recommend to read this blog article about DNA genealogy and what you can expect from it, to get an idea of how useful the results are you are getting. It's a very comprehensive summary of what is wrong with DNA testing in the US.

https://dna-explained.com/2016/02/10/ethnicity-testing-a-conundrum/