I really don't think you can plan it like that.
You can really just go for it when you feel ready.
People saying "a levels and GCSE's at the same time", "at uni at the same time" etc.....life just doesn't work like that and I'm always somewhat in awe of the people who seem to have blind faith that their lives will pan out just as they plan it.
Many kids don't go to uni, and that's ok. Many kids have SEN, or A levels just aren't much of a consideration for other reasons. Your kid might be off travelling the world as a pro footballer at 17. You might end up having twins.
There's just too many variables when you try and look too far ahead.
As you'll see from the answers here, many posters think they have the ideal age gap - and it's because that's their family and it's what they know and it's their life and they love it. But as at least one pp conceded, if they had another age gap they'd have found a positive in that too.
(For what it's worth, mine have just under 3 years between them. It's longer than I would have liked, for various reasons, but 🤷♀️ I just don't know how a smaller gap would have worked out and I'm happy with what I have).