In many cases, you never really had a 300m market in the first place. The EU membership spans a wide range of cultures, languages, traditions, economics and politics. Many products or services won't be equally attractive to customers all over the EU, particularly once you take into account the rules that mean you can't necessarily price according to what a free market would pay either.
If I look at most of my European sales they are in Germany, The Netherlands and Scandinavia. The UK was in that list until Brexit.
I don't think you need to target all 27, at least not until you are well established.