Last year when DS was 5 and in a play based K we homeschooled/afterschooled just for writing in order to keep him at grade level in that. It was a real struggle. We actually started off with more subjects and realized early on that being at a 4th grade level in some subject did not mean that he could be cajoled to become interested in a 5th grade level.
I don't have a lot of advice about other subjects but with the learning to write our OT (DS is very below average in fine motor skills) recommended at a K level that a single HWT page one to two days a week was plenty and that given how frustrating pencil use was for him that the other days be made more fun, such as forming letters with wood pieces, playdoh, drawing them in or with shaving foam, making them from food, etc. Even if he really already knew the shape that the consistency of daily exposure would be good in the long run. DS progressed very slowly it seemed and each day's work which ought to take about 5 minutes would range between 10 and 40 minutes for him. But now a year later he can write legible short words if he absolutely has to, meaning he's at an acceptable level from my perspective. Unless your DS has as poor motor skills as mine I would assume with a similar very small actual effort per week they would progress more quickly.
As DS progressed past the HWT K curriculum we've moved on to crosswords in his areas of interest... just google the interest and kids crossword. So that he was printing just a few words per day. Or I'd have him print out a label for his latest invention "Time machine" for example, and help him tape it on, and have that be the writing practice for the day.
We never got past battles about how long he had to work for or how much whining he was allowed to do before a time out. I hate to say there were almost as many meltdowns and tearful moments at the end of the year as at the beginning. We were firm and clear from the beginning and he resented it from the beginning. At least for my child there wasn't a period of testing or adjusting to the system that he got through after a few weeks.
With a child who really is bucking that hard against homeschool I would perhaps go more informal except in any areas that are truly deficient for age. Saving the seated time for those areas. And then also have one or two official activities a week that they must attend that you know they will find fun. For example perhaps there's a two hour block monday am and you do make a catapult that day. Something so that the anguish of the very small amount of forced labor the other days is balanced.
And then if you are going light on official homeschool time, additionally have an out of home goal oriented experience a couple of times a week: a group class of some kind where there are high expectations for them to behave well. A sport like karate, a language, a kids yoga class could work, etc.
Now at 6 I actually think my DS could handle a little more than the 10 or so minutes of unpleasant work he had to do last year.
I wouldn't worry that however much obedient seated time they are capable of at 5 is just going to be how it will be forever.
