no long-term motivation

It might be the best option really. I set it all to low (both AI and player) and while the car is slower to max out, I still got ahead of the other teams in R&D quickly. Probably I should not allow myself to adapt parts when a reg change happens and then have to work back up while restricting car development by not doing practice programs (other than race) and not maxing out the departments.

The game seems so close to being really good.

The main way I can think around this would be to start a career with custom settings and having the player increase levels set to low.  Maybe put the AI to high but it just means they'll max out quicker and it'll be boring that way too as the same teams would win every time as they all max out to their current level comparatively. 

You could also... you know... just not use them?   Like you have loads of money and resource points but you're under no inclination to actually spend them.    Just only upgrade to where you'd want your car and facilities to be relative to the others and then only use them to catch up when you feel you've fallen behind?    I get that's not the desired outcome, but you'd have more control over your enthusiasm levels until it's fixed to your satisfaction?