Excellent!
1k healing per minute is a good rough benchmark to reach with Mercy. That's not always possible though and depends on your team a bit.
What are your hero settings for Mercy under Controls? I use "toggle beam OFF" and "GA prefers beam target." I've tried the opposite settings for a while and strongly prefer it this way.
I only warned against doing competitive now because of my experience. I started as soon as I could and placed low, so it took me some time to rank up. But if you're not going to worry about rank for a while, it doesn't matter. And you're almost certainly better than I was when I started.
In terms of maps, Mercy is kind of weak on some king of the hill maps, especially Nepal Village and Nepal Shrine. Everyone plays too close together, so you're vulnerable AND your teammates are not far away enough to provide you a safe escape. I tend to play Moira on king of the hill. The only one Mercy is actually great on is Illios Ruins.
And yeah, Mercy is great on Route 66. (Similar to Blizzard World.) The second section of Route 66 is actually the most severe chokepoint in the entire game. The only way to solve that is to have your team run dive tanks, i.e. Winston and D.va. Mercy is a lot of fun to play with Winston and D.va because you can constantly slingshot yourself everywhere.
If you are getting flanked on Mercy too much, the only solution is to have a teammate babysit you, someone who's effective playing that far back, like McCree. The alternative is to play Brigitte or Moira instead, who can deal with flankers a lot better.
Reinhardt, in my opinion, is the most rewarding and frustrating hero in the game. I don't really enjoy playing him anymore. If you don't have a good team behind you, he's almost unplayable. Orisa is pretty great though.