Thank you for the good explanation, I will try to stick to the original part-numbers then, maybe 54 series instead of 74. If I find any optoisolator chips I might as well just change them. I am a bit afraid of any adjustment work needed after changing a lot of components in this monitor tough. There is a serious amount of potentiometers in this thing. a quick count amounts to 22 pots.
I am committed to fix this screen, I just got a NOS CRT tube for 100USD, shipping and customs will probably end up at more. But I dont think I will find one easily later.

SVD wrote on 2021-12-01, 13:42:

If I find any optoisolator chips I might as well just change them.

Be careful. At least care about the output type (photodiode or photo-transistor) and the current transfer ratio when you replace an opto-isolator. The current transfer ratio is often indicated by a trailing digit or letter. Don't ignore it. If you swap the opto-isolator in the mains power supply, that isolator isolates the monitor circuits from mains power. If you replace that one, make sure the replacement has the required isolation voltage (at least 1kV), and even better check for safety certifications (like "UL listed").

Thanks for the heads up on that issue too. Its really appreciated.
I have gotten a Blue ESR meter now. Its something I should have gotten earlier, its a fantastic tool. I will check the caps in the monitor when I have time. I might desolder some of the logic chips and test in my ti866II as well, but I expect them to be fried.