Carbon fouling spark plugs

Ive just had my engine compression tested: 125-120-165-125. Mechanic told me its still good for now but best thing to do is pull down top for rebuilt then check for other issues inside!
My car is a 1996 MAZDA familia w/B6 DOHC 1.6 engine, manual trans, power steering, no ABS.

I didnt had any major problem w/the car its very reliable & still performs good on the road despite its age. Except fuel consumption; 6 KM/Li w/AC on & 9KM/Li w/out AC, both city driving.

From June to present this year I already consumed 2sets of new plugs due to carbon fouling(all). Tip was dry(grey) then powdery carbon on isulator but when rubbed its maple like golden brown not wet w/oil or gas though. Ive already replaced air filter & had oil changed but still the same.

Can seafoam motor treatment cure this issue w/my plugs. Mechanic didnt approve this, saying it may cause some serious problems. Also he told me I might want to have my injectors newly replaced cause my engine is running rich due to my low fuel consumption. Im planning to have my injectors serviced instead, using ultrasonic system injectors cleaning. Will this be good instead of replacement. I computed, service will only take 2,000 while replacement would be 15,000-18,000!

Thanks much.

Regards,
Robert Smith