I was a SAHP from about the time my youngest was 2 until he was in Year 8 (13). I now work full-time.
No, I didn't get bored. I only get bored if I'm stuck in one place and have something I'd really like to be doing somewhere else.
My kids were all home educated for primary due to additional needs and very poor local options where we lived. I'm very glad to have done this. During the year and a bit after my youngest went to secondary school, we'd recently moved, so I focused on things around the house, volunteering including being a school governor, reading, taking courses, and taking on projects of interest on top of the usual helping and spending time when I could with my family.
I wouldn't say I wished I'd gone to employment sooner. It may have been nice if it had stayed as it was with my husband as the main carer for many reasons, but that's not how it worked and there are many things I would have missed, including the career I have now. It's one of those life would be very different, but likely equally able to have joy in.
I would strongly recommend if you're going to have a significant amount of time out to ensure you have the support to remain connected whether it's volunteering or otherwise - the work I've done since all required recent references or countersignatories, which I wouldn't have had if I didn't volunteer.
I don't know how to answer the question about feeling fulfilled with the hard work. I don't connect the two. I found peace and joy in it both when it was hard work and when it wasn't.
I don’t understand how you can still be a SAHM when the kids are secondary age or have left for uni? You’re a housewife, how boring.
For me, it was in part having disabled kids who needed additional support and part I viewed it like I was temporarily retired.
I've met people who find retirement irritatingly boring, some who returned to work because of it, but most people I know look forward to a time when they don't need to work and most retired people I know find joy in the peaceful calm of it rather than view it as boring. I view the time I had out of work with my kids in that manner - I was just lucky enough to not only have that peaceful calm where I could make my own schedule as retirees do, but to take it while able to enjoy it and have my kids around. I'm not sure I'll actually retire fully, I like having projects and the economy is unpredictable, and I'm not sure if when I get to retirement age that I'll have to health to do what I did then or where my kids will be. I'm glad I had that time when I could rather than do what I've seen a lot, which is putting it off, expecting the dreams to happen eventually...and then death or ill health beats them to it. I've had those dreams, and then when my youngest went to secondary, I started to build new ones.