Planning girlfriend visiting me in the UK

I'm a British Citizen by Birth. My girlfriend is a citizen of El Salvador. We are planning the future of our relationship and the possibility of getting married, however as we've only spend shorter periods of time together (2-3 weeks whilst visiting each other in our respective countries), I was looking at if it would be possible for her to come and stay with me for up to 6 months.

At the end of this time she would obviously leave and return to El Salvador, where if things go well we would intend to marry and begin the process of applying for the Family Visa.

She last visited the UK on the 21st December - 6th of January, and left on the 6th of January as planned. As a citizen of El Salvador she doesn't need a visa to visit the UK for up to 6 months (as advised by the home office website) however I don't know if applying for a tourism visa may improve our chances of her being allowed entry.

While she's here she would be staying with me at my house, I will be supporting her. I earn more than £26,500 and can easily support the two of us. My job allows me the benefit of working from home, and when I do have to go in to the office she would be able to stay with my family. We would also book the return flight before she arrived in the UK so customs and immigration can see that she does intend to leave. Chances are I would also book the same flight and return to El Salvador with her for a while.

I'm planning to visit her in June, and if all goes well then the idea was for her to be on the same plane as me coming back to the UK. I wondered if this may make things easier because I could go through immigration with her and explain the situation to the Border Control officer. I intend to be completely honest with the officer in question, and explain that we are intending to spend the time together to see if our relationship works over an extended period of time. After which, if all goes well we will return to El Salvador to marry and begin the process of applying for her Family Visa.

Any advice would be great, we're just looking for a way to try to make our relationship work right now. I've spent a lot of time reading the gov.uk site on immigration, as well as the advice given on Citizens Advice website, but I was wondering if anyone here had any additional advice or help?

Thanks in advance,