This is something I haven't found an answer for yet. Possibly the na is only added between tens and ones that are next to each other, so that would make it elfu themanini tisa ... but I'm just speculating.
"Elfu themanini na tisa" is indeed eighty thousand and nine. If hundreds, tens or units follow tens of thousands, then the number of thousands will take its normal place after the word "elfu". But if you go into tens of thousands as round figures, you should put the number in front of the word "elfu", e.g. eighteen thousand is "kumi na nane elfu", or eighty nine thousand is "themanini na tisa elfu".