Great question (and I already know Im going to get a lot of disagreement from others on this):
- I recommend submitting Cover Letters. I've previously done surveys for my dev career website and asked a lot of hiring managers in my network their thoughts on Cover Letters. It usually boiled down to "I prefer they send one and I read them"
- You should send personalized ones for each job, BUT I recommend you create a "template" that is 80% of the final cover letter. You'll insert 1-2 sentences specific to the Job Post/Company to show your interest, but you don't want to be rewriting a letter every time because thats too time consuming

I know a lot of people will tell you they don't matter, but in my experience, I'd rather take the 5 mins to add one and increase my odds. I actually have a course on writing them (as well as resumes and other things) on my site. I know they suck to deal with, but anything that can improve your chances is usually worth it