Can I just give you one idea to ponder, BeenBeta?
SATs are easy street for teachers. You teach 'em the tricks, they jump through the hoops.
All is then lovely until you get the same student for GCSE, & it becomes embarrassingly apparent to all concerned that your heavily coached 'level 6' student is never, ever going to get a grade B.
Meanwhile, his brighter best mate is kicking back, with a target of C - which will get her onto the college course she likes the look of - she only got a level 5.
OK, she got all of 2 marks less than her friend (& she had flu that day, as it happens) - still, it says '6' on his result & '5' on hers - her GCSE expectations are now one full grade lower.
Which means her teacher doesn't have to hassle her - she's 'on target' with her C, even though the teacher knows full well she could get a B. So she's left to coast - unless the teacher's actually teaching to the best of his ability to enable his students to strive for the highest grades they can attain. Most of us do our damnedest: tbh, I can't think why you'd do the job at all, otherwise.
With the target setting by SATs model, what incentive does he have to encourage a bright but not greatly motivated student to get out & push?
We've all taught the kid who couldn't be arsed until he hit year 11 & realised that his mates were off to college without him unless he got his finger out, & we've all taught the kid who was coached within an inch of his life for KS2 & KS3 SATs, & just couldn't quite hit an unrealistic GCSE target...
SATs, in & of themselves, do not make life difficult for teachers. It's a bloody sight easier to get cracking SATs results than it is to actually teach.
If I wanted to reduce my workload & make myself look good, I'd have SATs back tomorrow - to the detriment of my students' education.
I'd like to suggest that you try asking your dc's teachers how they assess progress, & what moderation procedures & safeguards they have in place.
Then do some research into SATs marking & moderation.
Then re-examine your faith in external assessment.