Problems with my Isuzu

I’m from the Philippines and we have a car from the brand ISUZU and the model is named "Crosswind"

Ever since we got it 2nd hand, it already had its minimal problems like slow acceleration and delayed shifting.

Fast forward 2 years, it now shows transmission slipping. Although my dad told me he had it checked as soon as he bought it. He told me he had the transmission fluid flushed and changed, but wasn’t there to watch the mechanics do their jobs. I’m a little judgemental when it comes to my fellow Filipino workers, because a lot of the mechanics here are scammers, well, around the world mechanics do really scam people who don’t have much knowledge about cars. Anyways, right now it shows some slipping and I checked the fluid and it looks clean. Right now my dad plans on "adjusting" whatever can be adjusted in the transmission, is it a good idea to do so?

I’ve done some research myself, the internet told me it could be fluid problems, but like I said, I checked the fluid and it looks good. On the other hand, I also found out about transmission "solenoids" but right now I kind of can’t check if our car’s solenoid is still good. Next thing I thought of is buying an OBD2 scanner since it could also be useful for our Toyota Innova which flashes the check engine light from time to time but disappears but what is weird is we don’t encounter any problems with the car other than sometimes weird sounds it makes when pressing on the brakes (The ABS light lights up too).

As of now, I don’t have any idea what to do because I base all of my theories on research and my dad on the other hand is a somewhat old school mechanic and he also isn’t sure what the main problem is. It’s a waste of money to buy a new transmission for an old car so we’re looking for cheaper solutions. Thank you for all your answers.