There's... so many threads on money-earning in GT7, but ultimate your answers are no, without much difficulty, and even playing in a relatively passive fashion you can do it in way less than a year.
Sure, doing the big-four races (I prefer Spa to Sardegna by far, though I don't think I've done any of them more than a couple of dozen times, and that was back before Spec II) are the quickest way to do it actively and a good way to boost up if you need it right now (say, for a car leaving Legends, and you don't want to wait three months for it to come back), but not really necessary.
Circuit Experiences give you about 50m for all-golds - I haven't checked the latest figure, but they literally just added another three (two were new circuits, which doesn't happen often) - which gets you almost 10% of the way to owning the entire car list but 90% of the way to CL50 which only requires a car collection worth 54,450,000cr.
If you're good, there's 104m available throughout the year for Online Time Trials (PS Plus is not required). Should you be a bit more average, it's 52m (nobody who plays regularly should be missing out on Silvers). That's another 10% for the car collection and you're way past CL50 now.
Weekly Challenges are a little trickier. On average, they give you about 1.4m a week (for beating five races, including the prize money for the races) but it's not always that straightforward. The third ticket can be credits or sometimes cars - though in either case that helps your CL, as prize car values count towards it (and you can sell the cars after for money to spend on more cars) and any money you get can be spent on cars. Sometimes you'll get a Parts ticket lower down which has no intrinsic value to the car collection. Nonetheless, that's about 70m a year in credits and in-kind, and you're at a third of the car collection and three times what you need for CL50.
You'll need PS Plus for the next one. There's various online championship running during the year (one right now) that give you a lot for not much, even if you're not particularly high-ranked, but they are relatively few and far between - about three or four a year. As you're given a ranking (and a reward) for your final position, you can rack up quite a few credits from only bare participation. Even doing a couple of races - which is a couple of hours - can get you 3m a season at DR C/D/E, and if you do a couple more to get a few more points finishes for a high rank you're looking at another 20m a year.
Finally, and much derided, there's the Daily Marathon roulette tickets. Once you're up above CL20 and no longer getting the lower 1-2 Star tickets, you'll draw in about 1m a month in credits and in kind (again, parts tickets aren't worth anything directly; you do get 1-2 cars a month). That's another 12m a year.
So just by logging on, doing 26.2mi a day (in your Circuit Experiences), getting Silver time trials, and showing your face in 3-4 races in each of the 3-4 online championship seasons, you'll be at CL50 in about six months.
And that's excluding the millions of credits of cars you get from completing the Cafe Menu in the first place. It shouldn't be more than a couple of months after you finish it.
gtdb.io
The problem is I enjoy the game and spend money "unwisely" hence keeping my CL in the mid to high 30s, and creeping up very slowly. Perhaps I need to be a bit more strategic with my money. I have the same issue in real life 
