Help with Head Gasket.

190K mile 2010 prius here. I've been taking care of my Prius and it served me for 5 great years. Changed the coolant, oil, transmission oil, and brake oil regularly.

Last week the abs brake booster and brake accumulator stopped working and after exactly 2 days the knocking started from the engine indication a headgasket problem.

I immediately changed the headgasket with a ginuine toyota head gasket and fixed the brakebooster and accumulator(off ebay from a crashed 2015 Prius with 55k mile for $170) . The booster was a great success but the headgasket is a different story.

The engine still knocks after the fix and through a micro pipe camera I could see liquid residue on the cylinder piston. Since the EGR and intake manifold have been cleaned, pcv replaced, new sparkplugs and coils, the only explanation the HG again.

I opened the engine one more time and removed the toyota HG and installed the Filpro HG (the blue one) and tested the car today and knocking persist but to a lesser extend and only when I start the car.

The cylinder head was leveled by the top machine shop in town, and I cleaned the mating surface on the other side with a sponge and paper towels without a single scratch (no razor). I tighten the HG bolts to specs (49 N+ 90+15 degrees).

Really don't know what to do. I can only think of the bolts need more torque but I don't want to break them and endup needing a new engine. I can also think of some little oil residue on the mating surfaces, though the paper towel should have absorbed that. But maybe when I installed the bolts that were dipped in engine oil somehow that oil got in between the surfaces. Has anyone been in my shoe? Please help!

I have some background in mechanics. Meaning I assume I did everything by the book but I might have missed something.