I will suggest :
Alec - I feel Alec is classic and timeless and transcends cultures . There is one spelling , and pronunciation. It also has some of the Grau tires of the other names you like - soft sound , has a C like Isaac , Callum Lucas but a less biblical tie ( Alexander is in the bible but not Alec) . I think it goes well with Emma and is also 4 letters and 2 syllables
Hugo - another name that I feel transcends time and goes so well with Emma ! 4 letters 2 syllables . And I love the meaning. I also think it’s a pretty international name , that works in all stages of life
Owen- maybe a little more popular than you had envisioned but I think it hits all the markers .
Otis- maybe little more on the trendy side but I think Emma and Otis are darling. I love that they both start with vowels
Reid - this one I think breaks most of your parameters but I think it works so well with Emma. It is only one syllable , and there are other spellings ( Reed) . But I feel like I haven’t heard of many but it’s also timeless