Some ND kids find secondary harder than primary. For some it is the other way round. And of course there are many who find both setting intolerable. It may be that one or both of your kids will like secondary even though it doesn't look that way now.
Regardless, home ed now gives them the opportunity to experiment in a setting where their needs can potentially be well met. For instance, if they like going to a certain museum but it is sometimes overcrowded, they can have the experience of going when it is less crowded. Or perhaps they will discover that they can get reasonable enjoyment from the crowded museum for one hour but no more, provided that the rest of the day is calm. So they learn more about themselves, how to meet their own needs, how to advocate for themselves. By contrast, at school they would just have to suck it up, because the museum trip is when it is regardless of crowds and lasts however long it lasts regardless of whether it's too much for an individual. So, ironically, home education can actually provide a better preparation for secondary school as it will have allowed kids to develop a better awareness of their needs and how to manage them.
If they carry on being home educated at secondary stage, I think you will find it more manageable than you expect. Before then, you'll have time to get settled in and look at slightly older HE kids and see what options there are. You don't have to know it all; you just have to be prepared to help them find the resources they need to learn what they want to learn. The parent's role is often that of researching, networking, and encouraging rather than teaching.
The subjects which my eldest focused on were ones I knew absolutely nothing about: art, music, sports coaching. They learned some of that stuff entirely off their own back, and in some cases I used the tremendously active and helpful home ed network to identify opportunities for them.
There was one subject (maths) which I do know about which they belatedly decided to study. Great, I thought: Finally, something I can help with!!! I have a maths degree and much experience of tutoring the subject. I will instil in my child a deep love for mathematics and at least a basic competence. Did they want my help to learn GCSE maths? Did they, heck 😂😂😂All offers were politely declined. These scenarios often don't pan out as you expect, but they do work out one way or another. They got the mark they wanted. As for me, well, I guess I could go teach maths to somebody else's kid if I feel too thwarted at home 😆