During Euro 2020 I have tried to test for you Lay The Leader (LTL) method (you can read more about it here). All the bets you could have tracked on my Twitter account. The test failed, but main reason was that I was not able to get enough data. It is not possible with small sample amount . In addition I was not able to get into the trade in all qualified matches.
I did only 10 trades, and below is the result:

My strike rate was 30% which means nothing with 10 samples. In Euro 2020 there was 51 games in total, and it is not enough data to conclude this LTL strategy. Much more data is needed which is why I am personally backtesting home leagues most often.

As you can see I’ve lost almost half point of my initial stake, which is not great not terrible.

Keep in mind that things that work in football season might not work on international matches.

If you cannot measure beforehand with proper number of samples, then you are gambling.