I've done the 'one weekend day is mine, one is yours' thing, to accommodate my hobby as it happens. It was pretty shit.
It results in no 'family time' at home, as the only family time that is accommodated is specific events, usually being invited elsewhere for some family do. Then there's negotiation over whose time that comes out of - your family, your time? No, that's silly. One afternoon 'out' leaves one and a half half-days each, which are then allocated. Endless negotiation, scheduling and a very eroded feeling of togetherness, as if we were only 'a family' for ceremonial purposes.
At a less intense scale e.g. taking half a day each at the weekend, then having a day (or two half days) together, or at least spent on family and domestic activities, so one person with dc while the other cleans etc. plus more flexibility, it's been fine.
By all means either get a hobby that gets you out for a bit, or just allocate yourself days or evenings off and go out - 'maintaining my sense of identity' or 'having a social life' are perfectly valid 'hobbies'.
I think though that your DH needs to recognise that small children are all-consuming hard work and that until they're all at least school age and have their own activities and ability to entertain themselves more (at which point you'll both become taxi drivers / activity facilitators), it's just not feasible for him to devote so much time to his hobby.
So, he needs to drop it for a few years, then go back to it later. He can still keep fit or whatever but in a far more time-efficient, fitting in around the family, way.