I think kickboxing has the lowest talent pool of of striking sports tbh, lots of kickboxers going to boxing and lots of older or retired boxers going to kickboxing.
The sports are too similar, kickboxing scoring is based on western boxing scoring.
If glory is getting stale for him, Holzken should take a few full Thai rules fights instead, just try to avoid clinching and find a way to deal with elbows, there are some promotions like Top King, Lion fight etc. that will take him, he could make money and still kick.
There's a long list of guys he could fight in Muay Thai if he's willing to vary his weight- Buakaw, Yod, french dudes.
His hands are good for kickboxing but going into boxing is throwing yourself into the sharktank with a huge wound on your leg.
If he ends up making bigger paychecks than he does at kickboxing, i'll be very surprised.
Hope he proves me wrong.