Being in the process of experiencing both, the restoration of a car and the renovation of a bathroom, I wondered whether there is really a difference between these two things. Well, to start with, there are clearly many similarities!
(1) In both cases you will find a lot of rusty tubes and major mistakes and shortcomings that were caused by people doing working long before you.
(2) In both cases you remove layers and layers of history to get to the real issues.
(3) In both cases you spend a lot (some people would say an insane amount) of money to establish a new base that should hold for the next 20+ years.
(4) In both cases a lot of the work done will be covered in later phases by paint or other cover materials.
There are some differences also, though.
Your family will love you more for what you do with the bath than for cherishing an old race car.
Talking to the car mechanic is so much more fun than discussing with the house renovation specialists. And finally, many more people will be able to see the result of the car restoration work than your renovation effort with the bathroom, for obvious reasons.
If you still need some good advise, then here's one: Don't try to do the two things in parallel and don't start the adventure if you aren't prepared for what may come up when you remove all these layers of history ....