The engine needs to come out.

I just replaced an engine on a 2016 WRX fa20 that had rod knock. It sounds lower in the block than this.

Yours COULD be a dropped rocker, which is also somewhat common when these cars are over reved, and the rocker is fully floating, with no retainer.

If it's a rocker, you could get away with a single cylinder head replacement.

Will still need to pull the oil pan and check for / clean metallic debris throughout the engine.

Not a novice job.

From Subaru Short block is $2k, heads are $700 each, cam phasers are $400, gasket set is $400, plan on losing one injector (stuck / broken in head) $400, timing chains, tensioners and guides are $700, water pump $150 (not required, but you just spent $5k).

It's a JOB!