I would take a few days at the start of each school break and spend the whole day planning for the term ahead. To plan my week, I usually assign a day to a subject and group, for example on the Monday I would plan my Junior Biology classes, on the Tuesday my Senior Chemistry, and so forth. I have 6 different preps (crazy, I know) and it would usually take me about 5 days to work through all of them.
My detailed planning is done in my Google Sheet. It gets a lot easier if you are teaching the same subjects every year. I would open my previous year’s planning and just copy and paste it into my new Google Sheet.