Thermostats are cheap, OEM is wahel brand or something like that, unless the water pump in it is some cheap off brand one then the only time I have seen the aisin OEM pumps fail is from the seals going and them leaking from the weep hole.
Are you unplugging your thermofan to flash tunes? I would check the 3 plugs into the fan relay, and if you have a wreckers close grab another fan relay to try aswell.
There is a xdf for tuner pro kicking around with radiator fan control settings too.
Changing water pumps is a lousy job and would be the absolute last thing I would try.
And I had 2 water pump bolts corroded and snapped off flush in the block when doing mine, this ended up being engine out to helicoil the block as access is non existent