Twins are never “easy” and you haven’t had it easy with their father either. Mine are thirteen months apart, so not twins, but about as close as it comes.
You are right about not being their best friend, that isn’t your job. And I don’t think that is what people mean by “enjoy it.” I think people mean that the whole thing moves so fast (particularly with twins) that if you don’t remember to stop and take joy from those moments when you can get it, you’ll miss it. For me, it was watching kids at a petting zoo. My son hitting his first triple, baking brownies with them when they were little and there was more brownie on them than ended up in the pan - those moments. And try not to dwell on the battle of wills that was potty training, the incredible temper tantrums, the number of times my son just got lost and wandered off.
As to ages, I liked my son, who is now thirteen, best when he was three and four. He was really a good natured, easy kid. He was a little too independent, wandering off by himself. My daughter at that age was a demon possessed child who threw incredible tantrums and needed her own way on EVERYTHING. He is now an uncooperative, uncommunicative sullen teenager pushing boundaries who I’m tempted to sell into child slavery - well, maybe not since we don’t actually ever SEE him unless he needs money or a ride - he is out with friends or hangs in his room and as far as I can tell, only talks in monosyllables when we do see him, she is now a lovely, considerate twelve year old. I suspect they will both turn out fine in the end, but he’s going to be a challenging teenager, something I would have never expected when he was four. She is going to hit hormonal here and enter the non-human stage, but I think compared to her toddler and pre-school years, she’ll be easy.