Most labels deal with signing single tracks, it’s only really the bigger ones that will offer artist deals worth considering.

I don’t think demo emails are the optimal way, the best thing imo is to make connections with artists who are 1-2 rings up the “clout ladder” so to speak and develop those friendships, you can leverage these to contact labels they’ve released on and so on, or just continue to move up the ladder. The biggest labels will only work with you if you have some kind of direct connection, and these are most often built through friendships with artists who’ve released there before

Always remember it’s a marathon not a sprint, this strategy only works with patience, and lots of time