Then if you do ENIG it kind of negates the benefit as the ENIG is very rough on the surface. so you are often better off with the solder mask.
Hard gold or ENEPIG is better but more expensive. best being bare copper, for a short while.
Just this to say, YMMV
If you play under 2 or even 3GHz ... Meh don't worry too much.
That, and the gold is thin enough that for some (surprisingly high, but I forget off the top of my head) frequencies, the RF current will end up in the nickel plating.
So from a pure loss perspective, not the best, but for consistency between board runs, no mask and using ENIG will at least be more consistent impedance/vswr wise than SMOBC, due to varying soldermask thickness or dK.
Can you get hard gold mid-board like that? I'm used to card-edge fingers, which require the bus bar that gets used to electroplate them, but the bus bar gets routed away. Not saying it's not possible, just not familiar with it.