I know that I'm certainly hoping that by the time we're havign kids I've started as a garden designer so that even if I get really sick and can't do a regular job I can design part time. I LOVE designing. Doing it for other people is the best as you get to try out ideas that you might not do at home! My garden plan for home is a disaster as I keep on trying to fit ALL my favourite plants in. I did one for my parents garden (based on what they said they wanted/liked) and it was soooo much easier, cos I just went "okay, this is what's wanted, this is the space, what can I do wih that?".
I think being a good designer is about having an eye for colour and spaces and then listening to people. Like, I LOVE your nursery, but your dining room isn't "me". HOWEVER, I can see that it's really stylish and the space and colours works really well . That's the kind of room I might design for someone else...in this house it would never stay looking nice so I wouldn't go for such a "perfect" design in the first place. Also, my parents got a pro garden designer to do most of their garden (not the bed I've designed) 3 years ago and while they love most of it, he totally didn't listen to what they said - blew the budget on 1/3 of it, etc. Listening to the clients is what makes people recommend designers to their friends!
And hey, if you did do that, you could use your engineer maths, technical know-how and perfectionism in something you really like that works around little Grant.