For your 1st question, all you have to do is become friends with someone and not best friends. Unless you are trading with someone that you don't know, and then all you have to do is put up fencing or terraform around your entrance so that they can't get any further than that. As far as your friends go, surely you are inviting people to your island that won't trash it. If not, then you need new friends. Lol!
As far as the 2nd part of your post goes, I find the game incredibly entertaining. But that's just me. I will admit, though, that I do time travel. I find it necessary to keep the pace of the game where I want it. However, I never time to travel forward. I always time travel back however many days I think I need so that I end up on the present day. Now, there were some times where I would have to time travel back almost a month in order to do what I wanted. Especially in the days where I was using amiibo cards to get the villagers that I wanted. I also time travel back in order to order things from the catalog so that I have them when I want them.
I would also comment that I find that this game is way more fun playing with a group. There are 3 or 4 of us that play together and help each other out quite a bit. It has made the game way more fun and engaging when I find some inspiration and all I have to do is ask one of my buddies for a piece of furniture or a DIY that I need. It's also nice when we are trying to get certain things in the game and all 3 of us can grind that out and it doesn't take so long.
Nookazon has also changed the game for me. It allows me to buy or trade for the things that I don't currently have. And, if you have cards you can use those to barter villagers off of your island in order to get materials or tickets to buy other things with. That's what I spent about a month on myself. I would bring villagers in and then sell them on Nookazon in order to get the currency I needed to buy the other things I wanted.
All in all this game has been a blast for me. I'm about 360 hours into it which is immensely more time than I've ever put into a single game before in my life. However, it wouldn't be near as fun without the friend group that I've put together. I would encourage anyone to do the same thing we did, which is find a few friends that have similar play interests and play the game together. It's amazing what a few friends can do for each other.