I think you just need to chill. If you have an able child they will get through the 11+ exams as long as they have some practice behind them. We started in March with DS doing the 11+ (and other selective) exams twixt early September and mid-November of the same year.
He wasn't the most willing of students so we certainly didn't 'over-do' the prep - he was doing Bond-online with half a maths/English/VR/VR test every couple of days too right up until the summer hols. But we didn't teach him per se - it was practice and just going over the questions/maths he didn't seem to get.
FWIW I think there was a reasonable amount of maths that they hadn't yet covered in KS2 syllabus (by the beginning of Year 6) but some schools might be more ahead than others on such things so you can only really find out thro' practice.
We slightly buried our heads in the sand about DS's English (wrongly so with the benefit of hindsight), so did not really concentrate on essay-writing etc....However, it became evident that when DS needs to, he is quite capable of pulling a good essay out of the bag (just that for most homework at primary school he couldn't be bothered to!) so all was well.
DS is now at a super-selective. I think what clinched it for him was less the six months' practice of the different components than speed and not getting stressed about exams. They are however less easy to 'teach' than the core skills required of the tests.
I think that the 11+Forum website is scary indeed. Some parents seem to expect the earth of their DCs in preparing for the tests. It is really not fair to put 10 year olds under that much pressure and the sense of disappointment is no doubt ramped up significantly, the more there is riding on a positive outcome :-(.
As DS passed three 11+ exams we are considered to be 'experts' by other parents at the primary school our DCs attend(ed). We are not. I would certainly not sanction prepping them for years in advance. I think six months to a year is long enough. Certainly by the time DS came to do the 11+ exams he was really fed up with the practice (even though it still constituted only about an hour of his summer hols' days) and was making loads of careless mistakes in his papers. As we all know from our own experiences, one reaches saturation point and anything thereafter can confuse and be counter-productive.
What I would advocate is starting with the Bond books and then moving to other worthy exam prep closer to the exams. It also helped that the Susan Daughtrey prep books and others had shorter tests in them so DS saw them as more appealing (and was therefore more receptive to them) in the final push to the exams!
Good luck and I don't envy any of you. We have a 7 year old DD who has all this ahead of her..... ;-(! And we've worked out that she will be prepping for 11+ as big bro' is doing his GCSEs!