Congratulation on the meter, it's a great unit.
On the 4th picture from the top, there is quite a bit of flux residue. This should be cleaned. If there are other messy spots like that, they should be cleaned as well. Of course, look closely for bad solder joints and bad contact with connectors. A usual problem with these units is the electrolytic capacitors. They tend to leak and cause all sort of issues with other components and the boards. Some people say they had a bad batch of capacitor at the time even from reputable manufacturer. A complete recap is probably in order then see if the problem is still present.
As for the front panel, something that worked very well for my 2001 and my 2015 is bleach. This video shows a method that worked for me at least.
12% (40 vol) peroxide cream works well because it sticks to the surface. You can then wrap it in saran wrap and let it outside exposed to the sun for a few days. It will not become completely white, this is not its original color, but the yellowing should mostly disappear.
https://xdevs.com/ is a remarkable site that contains a wealth of knowledge and data on these devices, I suggest you take a look if you have not already done so.
Be cautious when working on these boards, make sure you use an ESD wrist strap and don't touch the components too much if you don't have to.