Riv,
Its a common story unfortunately. Remember, these bikes are four decades old, and many were considered "disposable". That plastic was not well cared for in most cases, and the UV rays have done their damage over time.
I recently picked up a '75 CB750 in Planet Blue - 100% there except for the stock exhaust. When I went to remove the side covers, they both literally shattered - and I was being VERY careful, spraying some lubricant into he grommets. I will be replacing them with repops...
Look at this way, if you went through the trouble and expense of painting them, and then they cracked, it would REALLY suck!
As for you plateaued progress - take a breath, make a list of what your next steps should be and start knocking them off one by one. Soon enough, you will be moving forward and feeling satisfied.
Keep it up!
Gersh