I'll give you some tips and I was ranked under 900 and part of the NEPL but quit competively last year and now with the quarantine, longer. I want to get back into the league again however. Here is some tips I have learned personally and from others.

1) Not sure how common it is but I have "completed" many games.
2) You generally improve year after year and I would say plateau until you discover a breakthrough technique and exploit that to get to the next plateau. Personally though EVERYONE wants to be Keith Elwin or better but set expectations, lol. Some people are just born better.
3) Eh it takes about 15-20 games, not sure.
4) I would say you do in general get worse over time and I'm 32. Like for me personally I used to play competitive videogames and I can't do the stuff I did when I was younger. Now that being said you look at Bob Mathews and other people and they aren't young either.
5) I....I feel REALLY hesitant to answer this. Like there is a plethora of really good female players. Ok. So to me women players aren't as accurate as male players and add to this they struggle to nudge. Ok there I said it, roast me. Seriously though there are PLENTY of seriously good female players.

Ok here are some of the tips I have observed that you don't normally see posted online or talked about.
1) To get better accuracy put your eyes level with the flipper and angle your face with the flipper while a ball is trapped. Close an eye and line up the ball with your shot. Flip when the ball is at your shot and over time you "train" yourself to be better with accuracy. It's a lot like shooting with a rifle. Observe Lyman Sheats and how he plays.

I wouldn't recommend playing sitting down however others have suggested this. My reasoning is that you normally don't play pinball this way and it will mess with your accuracy always rather than learning to be more accurate on the shot you want at that moment.

2) Learn to be aggressive with the game but not be a dick to the game. When you are in a tournament in like finals or something nudge to win! Accurate nudging that is light often is better than major nudges. If you are nudging heavily a lot you have poor accuracy. With EM's though nudge as much as you can in the pops. Also FORWARD nudge on the outlane is always better than a right or left.

3) Ball trajectory prediction. This comes with time and transfers across any game ever made but you can judge when a ball is in trouble as soon as it comes to you. This is affected mainly by the speed of the ball and your age.

4) Where you are located has a major influence on you to learn how to play competitively. I know hands down PA and NY players are better than NEPL folk. Our top players can beat them but a lot of PA/NY players are just outstanding. Certain games are located more in certain regions as well.

5) Giving up early and saying it's a "lost cause". There is a high profile player here that recently I noticed stopped this and has become markedly better because of it. I doubt I can beat him now because that was his major flaw aside from EM's. I have beaten Bowen less than a handful of times but it was purely luck. The best players never give up. Bad ball 1 and a tilt on ball 2? Well they somehow beat you on ball 3 because they didn't give up.

6) Play to win. Always. I believe there was a podcast with Joe Lemire on the Pinball Players Podcast and he mentioned even playing casually he plays to get the biggest score. A lot of the more high profile players have joined Stern recently and while that's good it's also a tinge bit bad. Pro players focus on tighter shots with harder rules because anything less doesn't challenge them which isn't fun. Too easy and it's not a fun game and is easy exploitable. You will see a trend in pinball that won't be stopped because tournaments and leagues are becoming bigger and there are better and better players. The King of Kong documentary is ridiculously funny that I'm shocked people thought it wasn't scripted. Certain elements though you will find very similar with pinball and sadly you will find out to get better at pinball like Billy said, you have to pay the price. That price is money and time. It's not often fun but it is fun to get better or hit your shots, etc. So to get better at pinball you really have to exploit the rules and physics of pinball which in part destroys the fun of pinball. Your making it into YOUR game and controlling it rather than the game controlling YOU.

7) You will get better purely not knowing it or giving up. Give up learning how to live catch and let the ball dead bounce. It's how I learned after being frustrated learning how to live catch and learned a new technique. Dead bouncing is a MEGA huge plateau that will make you vastly better and even the pros struggle with it instead opting for live/drop catching. Certain things you will get better at and others you just can't or will ever be able to do. I don't think Bowen can slap save for example.

8 ) Ignore any tutorial out there and learn how to play a certain game your own way. A lot of players will follow a PAPA tutorial and certain things you have to do in that game for the big points. Well do that for big points (you can't ignore multipliers on Star Wars for example) but develop your own strategy and play that strategy during casual times. Don't be afraid to change your strategy though if you missed so many times and just go straight to the multiball! I have seen tons of people go for that perfect Monster Bash mb and you just crush them by taking 1 monster into mb instead of like 3. This is a major one in Pinburgh where you just want to survive for every point.

9) Play often and play a lot. Also if you are draining in the center you are playing well. Outlane drains means the ball was out of control meaning you had poor control of the ball.

10) Expect to be really good around ~5 years or so. It took me that long at least.