S90C is a Quantum Dots OLED TV , pretty much the only type of TV where Quantum Dots yield HUGE benefit , it is significantly better than any Mini-LED TV in terms of visual aspects.
Or if you can find a S90D at similar prices which is just as good.
Unless you are ONLY going to watch just 1 specific sport / news channel with a HUGE logo,you also have nothing to worry about.
Most of the people who got Burn-In from OLED are ppl who have bad watchings habits such as :

1) Away from the TV to speak on the phone for 3 hours.

2) unplugging / turning off the power supply for OLED TV screen is off. OLEDs need to work their pixels magic in the background when powered off , you don't turn off the TV completely unless it is for reboot once in awhile.

3) turning AUTO-HDR on every type of content , even for non HDR games / movies. Not only this is entirely pointless, it is actively pushing the TV limits ( HDR demands 100% brightness values from the TV ).

4) Mistakenly bought a used / second hand OLED display that was already being exploited for thousand of hours .

Otherwise for the normal guys who play a variety of content such as streaming , playing different games ,  the modern OLED are very resilient towards Burn-In.  ( I have 9 OLED TVs now , none of them have burn-ins . 3 of them have 12k hours + watched) .

You can unplug the TV during the thunderstorm. Just don't do it everytime , allows the TV times to finish its business .
Get one of those power surge protector.
As for the warranty , it depends on if it covers dead pixels , burn in and other issues. If it does , 3 years is still a good deal.
If your TV is broke during that 3 extra years and the TV model / sparesparts are obsolete , they may replace your TV entirely with a newer model.
Again though you also want to check if the warranty is covered by the shop , or by the brand.  If it is the shop I would say no .