I've maintained a range of hobbies through motherhood and it's had a range of benefits to me and to them. Some hobbies were independent, some were done with children in tow when they were younger.
I don't fear empty nest in a few years when my teenagers go to uni or otherwise become more independent and use less time.
I am fit and strong improving my odds of a healthy, independent old age (which potentially benefits my DCs in their adulthood)
I am a positive role model. Parkrun started as my hobby, and I started taking my DCs to junior parkrun a decade ago. They're still running with their own goals and run for themselves at an age when many teenagers drop out of casual physical activity.
My volunteering helped me return to work after a prolonged career break. I had something relevant and continuous to put on my CV. When my DCs were young, they could accompany me and that broadened their experiences.
My sense of self has been continuous despite changes in circumstances. I am a complete human, not just mummy. It's been easier to adapt to my children's shifting needs. I am more interesting to myself and other people.
DH has been a fully involved parent. Even from 4m old with EBFed babies, he got time in short windows to parent in his own right even just for an hour between feeds. By a year, he got an annual weekend with them. I've never had to fear leaving him to parent. He might not do it the same as me, but that's OK and not worth sweating over.
I use their time carefully. When they're at their activities, I tend to take a book to read, and used to use it to sew their Scouting (and my) badges. Being out of the house, it's easier to find flow to concentrate without the sense of obligation to do other stuff. Staying in the waiting room or going somewhere nearby like a pub cut wasted travel time going home and back and achieving little in the gap.
There were times when they were little that I might end up with a toddler in a pram on a run so I could still fit my long run training in around work and childcare. When they were very little it was a bit squeezed in when DH was avaliable around long work hours, but lack of other support made that me time even more important. As their pressure on my time has eased, it's become more natural to fit my life in.
There's a thread running where OP is struggling with the family balance and adding in a new baby to difficult circumstances. Part of the problem is that she has no other facets to life beyond motherhood, and she doesn't let go enough to let her partner be a fully involved dad. It's not just about cute babies and toddlers, parenting is a long game of a couple of decades (and beyond) with an eventual outcome of raising functional adults.