I did O-levels and A levels, taught GCSE and A level, have DD in S5. Moved to Scotland halfway through primary. There are good and bad elements in both systems. I'm glad I got DD out of the new Gove English system in primary and she did much better here, but I do think exams over two years at the end of secondary is a less-pressured system. I may change my mind again in S6 when the pressure is off.
They do study a book. My DD would not hate The Great Gatsby as much as she does if she wasn't doing Higher English 😂. It's worth noting that English is two separate GCSEs in England, both taught over two years, whereas in Scotland Nat5 is a combined language and literature exam taught over 1 year. Of course, Nat5s and Highers are one year and GCSEs and A levels are 2 years, giving more than double the learning time because there's no pause for exams within that. So obviously they won't do as many texts at Nat5 and Higher as at GCSE and A level. Doesn't mean the level is lower in what they do, just that it is less broad.
Depending on where you live, you might not even be able to do three AH in S6. DD can only do two next year because her school doesn't offer them in her other 3 S5 subjects, and it's not even two she would necessarily have chosen had others been available. I thought that was a small rural school issue, but I asked about it on a thread here a few weeks ago and there were posters in the same situation in urban areas. Posters also confirmed only 2 AH plus Highers needed for university in England.
GCSE is broader than Nat5, but not more difficult. Some topics not taught in S4 are taught at GCSE and these may come up at Higher, albeit at a greater level of difficulty.
Every school is different, but one big difference for DD's school is homework. Barely any at primary (rightly imho), compared with six pieces a week in England, right from Reception. Hardly any from S1-S3, and mostly done on an app on the school bus. I think it would be better if they had more at this stage. Then a mega switch is flipped in S4 and the amount of home study required is a huge shock to the system. DD was burnt out by the time exams happened in S4 and had no energy left for the final revision push and I fear this year will be even worse. Transfering from GCSE is probably less of a shock as they will be used to doing a lot of work at home. Only doing 7 subjects from S3 and not having to do a science or language beyond S2 is another big difference from England. There is an expectation that all pupils do Higher English if they want to go to university, regardless of subject. I've heard some schools also insist on Higher maths.
Agree with other posters that S5 is the right route. There is a thread on here where someone was in the position of having to fight for their DD to go into S5, as their age put them in S6. I read it last week but not sure if it was a recent or old one.