> for instance a Dutch person would spell 'mijn' as 'm', 'ij', 'n'

Not anyone under the age of fifty, unless they want to deal with the follow up question too. ("Is that a long or short ij/ei?")

Here's a screenshot from just one random Dutch children's spelling quiz online:

https://postimg.cc/47wFHy0v

Separate letters for 'ij', despite it formally being a single letter.