Ahh the old, the mainboard has failed. This happened to my own Sony TV, different model from yours at
the grand old age of one soon after a software update was applied.
Luckily my TV was still under a 5 year warranty so repair was free, repair person said it was a common fault for Sony and the part is about £140-00. TV is now three years old works flawlessly and had two further software updates and survived.
Don't know if the price you were quoted reflects a world shortage of parts, your TV is from 2015 so that part has been lying around for a while, you have to factor in labour costs. On any piece of equipment any price you pay for a component will always seem excessive compared to the retail price of the full unit.
This is the time of year when TV manufacturers introduce their 2022 models.
As the new models go on sale you should see the prices of last years models reducing
you could play a waiting game and see how retailers like John Lewis or Currys are
discounting last years models. They both offer 5 year guarantees.
richer Sounds offer a 6 year guarantee.
Often the new models are just last years models with a new case and stand, shop wisely.
