Buying a new Z370/390 motherboard just to be able to overclock 8700k won't be 'worth it's features in gold'.
I mean, it won't be worth the $100 - $200 you may end up spending on the motherboard - even more, if you current cooler isn't able to keep up with that extra heat from overclocking.
It'd be a different story if you'd have gotten the Z mobo from the get go.
Roll with what you have until you're no longer satisfied by the cpu's performance, and replace the entire combo(cpu/mobo/ram).