You have to look at the bigger picture. Yes, you may find good sex with someone else, but is that enough? How would you feel to be a single parent again, what about the financial implications of breaking up - could you afford a house/lifestyle on your own? How will it affect your children long term? Can you cope mentally with a "blended" family if you found someone else you do fancy and want to merge your families? All practical stuff really. Sex is only one part of the bigger picture. You have to be pragmatic. It's highly unlikely many people have "everything" i.e. lifestyle, partner, family, etc that's perfect for them. Most people make compromises. It's up to you whether a compromised sex life is worth everything else your DH brings to the table. Or whether you'd prefer a better sex life and compromise on everything else. It's all a matter of balance.
You seem to suggest that you're happy with DH in all other aspects. Are you willing to give that up for better sex? Only you can answer.
Your alternative I suppose is an affair or FWB kind of situation where you get the sex you need but you risk your marriage - if he ends up finding out, your break up/separation/divorce is likely to be harder and more stressful and more expensive than if you separate amicable before you get sex elsewhere.
Another aspect is whether better sex will make you happy for the longer term, or whether you'll be "bored" with your new man after a few years and be back in the same position.
Nothing wrong with fantasising and playing etc - most people do it. Most people will close their eyes and think of someone else during sex during longer term relationships. It doesn't necessarily mean you have to scratch the itch.
I've been in your position and basically didn't have sex or any other intimacy for many years during our 38 year relationship. But I didn't have sex with anyone else either. Yes, I fancied other people, fantasised, etc., and wore out my small selection of sex toys. We just didn't/couldn't do anything with each other for a few years for various reasons. We've got through it and started being intimate again and things returned to a mutually acceptable/enjoyable level of intimacy. In my case, I think it was easier to accept the potential loss of my sex life when middle aged because to an extent I'd filled my boots when a lot younger, so had "done everything, tried everything" with my first two boyfriends (the second who became my OH), so didn't think I'd missed out of anything.
Sex drives for a woman ebb and flow over their lifetimes. I'd have to think very long and hard about giving up an otherwise perfect husband just because the sex wasn't as good as it used to be! But that's me and everyone is different and has different priorities. It's got to be a balance as only a tiny number of people will have perfection in all areas from their partner.