I'd recommend (strongly) to drop in the odd word spelled phonetically, and leave it at that, perhaps allowing dialogue tags or narrative to deepen it.
The beginning of my WIP has a Geordie vicar. My family is from the Northeast and I'm familiar with the Geordie patois but to write it authentically? No way. It would be inscrutable for most - if I said 'divvent dunshus' would you be able to parse meaning - probably not.
But perhaps more of an issue for me (I'm the kind of reader who is happy to stop reading and go off on an internet search for a word or phrase I don't understand - or even a scientific principle), is the annoyance factor. I recently read Hard Times by Charles Dickens and at the end a lisping character has a lot of dialogue. Pages and pages of it. It drove me mad. Even in Great Expectations, Joe uses w in place of v's and although it's rare, I still get irritated by it.
I think we aim for verisimiltude in our prose, but on matters like this, it's important to favour the reader.
pH