Should I change my oil every 6 months or once a year?

Similar use, I changed mine every spring

 

More extreme case but anyway: for our Canadian spec gen 3 Toyota Canada says every 6 months or 8000 kms (roughly 5000 miles), which I followed for about 12 years, while our usage kept dropping off. I would change it every April and October. About 3 years back, one April, I changed the oil, and while entering it in my log, noticed it was barely 1000 kms since the previous change. Year 'round we're lucky to do 3000 kms. Resolved then to go to once a year, every April.

For your case, even Toyota USA stipulates 10k miles or yearly, though a lot of owners half that. But considering your usage, should be fine.

 

The OEM manual states 10K miles or annually for oil changes. Your obviously not hitting the 10K mile mark; so pay attention to the time related footnotes in the OM.
As a side note; there is no change interval for the ATF or brake fluid; but you use both every-time you drive the car. From a basic maintenance standpoint; ATF should be done every 60K miles or 10 years and DOT3 brake fluid on other manufacturers are 3 years or 36K miles. If you want to get cheap about it, I'd do it every 5 year or 50K miles minimum; to avoid issues. Cheap insurance.... There's lots of people on this forum that don't do either and wonder why they have brake & transmission issues. My approach is to spend a little up front, in hopes of avoiding a large bill later. In my lifetime; I've only had a brake booster go out on me in a VW - it was covered under warranty. Needless to say I now avoid VW's.

 

Just editorial: in the States, maintenance is listed in the Warranty and Maintenance Booklet. I haven't done a gen 5, but here's gen 3 summary I cobbled, spreadsheet and pdf, a little easier read. It's likely very simiilar, and you could use the spreadsheet as a template, adjust to suit. Also, extrapolated to 240k miles or 24 years: