Let's put it another way, would Americans be happy to have a first lady who has never lived or worked in the US? Melania Trump is certainly not to my taste but at least she lived & worked in the US for a number of years prior to when her husband became president.
Obviously I don't expect Meghan to live in the UK for several years prior to an engagement but at least for a period wouldn't hurt. And it makes better optics in my opinion. How can you be a representative of a country that you have never lived in? The royal family are funded almost entirely by the British taxpayer from their security, to where they live, travel, holidays, their clothes, their staff, nannies etc.
Britain has become more insular especially after Brexit, so it would be risky if they were to announce an engagement just as soon as she moves to the country. Think about it, someone who has never lived or worked in the UK, suddenly arrives & announces an engagement where she will then be put on the British taxpayer payroll upon her engagement? I dont think that will go down well.
If you look at some of the other foreign spouses of other royal houses like Princess Mary, she lived in Denmark for a year or so before an engagement was announced.