Seo for keyword in domain name with lil backlink

Hello Warriors!! I was a small blogspot blogger few years back with an adsense account. My adsense is still active although I havent used it for many years. Now I am willing to make a blog and monetize it with adsense. I want a make a general blog covering a particular broad topic. But what I see the recent changes are most social bookmark, blogs etc are no follow. So it will be very difficult for me to get backlinks. So what I feel is if I do niche/micro niche blogging and buy domain with keyword in it will that help me in search engine ranking for that keyword??? Say eg. General topic Donut, I buy niche keyword domain name "Chocolate Donut" , so will it help me rank with Chocolate Donut term???

Secondly, what if I buy a general domain name say "Donut.com" and make subdomains say "ChocolateDonut.Donut.com" "VanillaDonut.Donut.com" etc so how will the keyword in subdomain rank in search results???

Thirdly I have got some good .info domains. But as generally .info domains and considered spam so how will this .info domain rank in search engine against a .com or .org.??? Even if it ranks fine in search result will the visitor basically not visit the page because its a .info (spam)???

I am not looking forward to make spam site. I want to make a really nice blog with good quality content. How much only quality content with very less backlink help me rank in search result??

I would want to make blogs abt health related topics but what I see is competition in SERP is fierce in Cancer/Diabetes genre. Is it at all possible to rank in say Cancer -> Ovarian Cancer niche?? or say Diabetes -> Type 2 Diabetes niche??

Thanks a lot.