And indeed, they were complete and utter generalisations. I'm sure there are people in interesting jobs, who come home and are able to work on interesting side projects. Either because their jobs are interesting but not brain-draining, or because they are particularly driven, or any number of reasons.
I have just found that at this point in my life, those are the two prospects that I am weighing up.
3) Use your R&D skills to create a useful technology or service in an area that interests you and either license it or found your own company around it. Quite a common route for academic types.
I'm familiar with a few people who have done this, and they really enjoy what they do. I wish anyone doing this all the best.
However it suffers the same problem as 1). That is, you may not have the energy for other projects. I understand the sentiment of your suggestion, which is: "My side project is also my work", but then you are committed to that project for X amount of time. Whereas with a typical side project that I might play with on the weekend, or with open source contributions, I am free to dabble with whatever interests me on that particular day. It is this freedom and flexibility that I am seeking in my life (partially because I have the attention span of a newt :)
