There are a lot of ways to go with this but the most obvious in my opinion is to form a company and do consulting/dev work through that. It seems like you want

There are a lot of ways to go with this but the most obvious in my opinion is to form a company and do consulting/dev work through that. It seems like you want to continue making money from engineering in addition to your day-job, the only way to guarantee you're making a quality wage with flexibility is to go on your own IMO. Advertise yourself as having a lot of experience and being good at what you do and target folks who want someone to "jump in, do the job, and jump out". This may be more overhead than initially desired but certainly more satisfying and longterm sustainable than Upwork or some other random gig company. Other than that selling things you own (bikes or whatever) is probably the easiest way to make "cash" but seems you want more than that. Good luck to you Edit: a lot of comments talk about saving more and while that's higly relevant it's not something OP mentioned in their post