I was measuring every leg where it was soldered to the board from below.
And that is measuring everything connected in any way to those legs. So if there are circuit board traces that connect one of those legs to capacitors, chokes, transformers, etc, you are measuring the impedance of all that stuff, not just the impedance of the FET.
Now this doesn't mean that you can't discern anything from the measurements. In fact, one of the tests after a circuit board is assembled is a so-called "bed of nails" test where probes contact many different points on the circuit board and measure the expected impedance. As I recall this is done without powering up the board, just as you are testing. The expected values are all calculated from the combination of the circuit design and the components used so the tester knows what to look for. This is done by the software used to design circuit boards, not with a pencil and paper. There are hundreds, if not thousands of test points.
You mostly need to understand that the in-circuit measurements can/will be different from what you would measure if the FET were removed from the board, and can therefore be misleading if you aren't aware of this.
Over the years I have attempted repairs of a variety of electronic equipment, almost always unsuccessfully. And I've got a degree in electrical engineering. The last attempt was an inverter/charger where I diagnosed the problem to the board level, and bought a replacement board. Put everything back together, applied battery power, it turned on which was great progress, then a few seconds later went Pop and burst into flames. I was then out the cost of the inverter/charger/ plus the cost of the replacement board which was not cheap. This is typical of failures where there are multiple things wrong. You find and fix one, but when you power it up the other faulty components cause the newly replaced components to fail once again and you are back to square one.
A few people here seem to have more hands-on repair experience than me, and certainly more success than me, so I think you are getting some good guidance. That said, I think the likelihood of success is very low. Even if you clearly identify bad components, removing them will require a high quality soldering iron and a good solder sucker, and even then it's really easy to burn or damage a thruhole. Many of the components are big so easy to work with, but there are also a bunch of surface mount components on those boards, and they are much harder to work with. Some even require specialized equipment. And component part numbers are often obscured, or not present at all on smaller devices, so figuring out what replacement parts to buy can be a real challenge. Only you can decide how to best spend your time and your money, but keep this in mind as you weigh the two against each other.
