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Now my car is older I have tended to do it every couple of years, and this year did the minor one myself (plugs, oil, oil filter, air filter, pollen filter). However, the next one I'll take the hit of sending it to a main dealer for the major service and a full check up.
My MINI had some sort of extended service package so I originally followed that, but they were very spaced out* and for the price I started taking it to a bmw indy specialist annually who was better imo. When I swapped it with my folks they went back to following the BMW service packages, mainly because they'd collect it.
My KA before it used to get done annually.
I your case I'd just get it done every year or two and keep an eye on the fluids or yourself. Idk about the Yaris but a basic service like I did on a Golf isn't that hard. However, without a decent area, jack and axel stands the oil change is a real pain. Frustratingly so for what is technically an exceptionally simple job. I know people will explain why the oil suction do-das shouldn't be used, but for diy not having to lift a car high enough to drain oil would be a game changer. Also plugs can be a fucker. But the filters are just unscrewing and opening things, pulling one bit out and stuffing another back. As for plugs Imo dealers recommend doing them annually because they're quite cheap and can be done while the oil drains, and look like something has been done - I can't understand how they can wear out that quickly.
The excellent wechillanycar guy said old cars will probably need a regas every 2yrs. Ours has needed a regas twice in the last 4yrs so it tallies. Very annexdotal, but IME Japanese cars have much better air con than European cars.
*one aspect of the Prince engine failures you read about I'm sure
How often do people actually get cars serviced? Just looked and it's ~ 3 years since I last had my 2017 Yaris serviced. Mileage is pretty minimal, 3k a year. I don't have any intention of selling until resale value is minimal so not too concerned about that aspect.
I've done nothing to it other than MOT in those years, is it worth a service? Similarly, it was 4 years ago when I last had the air-con regassed. Does this need doing again, cold air still seems to come out the vents? I've not noticed any issues whilst driving.