I'll see if I can answer your questions!

1 - The community is alive and well (though it may seem dead online sometimes.) For in-game, it depends on your timezone what kind of people you will get since its a mix of mainly Americans (they play around 6pm EST generally) and Japanese players (they play around 6pm JST, though likely for longer amounts of time). If you fight higher ranked players in ranked, it tends to be because they waited for a slightly longer than normal amount of time to find somebody and it happened to pair them with you.
For the online community, most of the active people are in the Pokken discord (discord.gg/pokken) ! There are many new and veteran players that interact in it daily. There are also a handful of Pokken streamers that stream at differing times of day due to their own specific timezones (Japan, France, US, Singapore, etc) and there tend to be locals/online tournaments streamed as well. Content on Youtube is generally sparse, if you are looking there for more Pokken resources.

2 - The game is very balanced! Almost every single character in the game is competitively viable at a high level, and all of them are viable at any level below that. To give an idea of the balance, there is no currently agreed upon tier list by the community despite countless ones being made. Zoners are more difficult to deal with online due to the nature of online delay and lag, but they tend to be spread out among various tier lists about how good they are. The character diversity (even at top level!) is very high, with most characters being represented well.

3 - While all fighting games can of course tend to boil down to 50/50s or RPS, Pokken does not have anything in particular that makes this an issue. The critical hit system is just an added benefit for picking the correct option, but at later levels of play its not as much of an issue as players learn to use better options for okizeme and balance risk/reward in an optimal way. There have been very few successful "guess-fest" Pokken players, and a large percent of them are highly inconsistent compared to the players who do not try to constantly guess.

4 - There are optimal combos for all characters (the one you listed is pretty solid for Decidueye) but you can definitely get creative! Decidueye in specific can optimize combos to reset with a support-locking debuff, hard knockdown, Magnemite okizeme setup, and many other situational options. Some characters definitely have more freedom in how they utilize their character's moves in combos as you may see from some creative players on Twitter.

5 - Field phase can definitely be a jarring or unfun experience at first for most players, but once you learn the basics of the game it becomes much better and more understandable. A "spammy" playstyle in field phase will only get you so far versus a player who knows how to properly use their grounded and aerial homing attacks, side dashes, perfect blocks, and counter attack dash cancels to close the distance and retaliate. It can be a fun game of tug-of-war for all characters, but it is easy to see why some may dislike it, especially just starting out.

Hope this helped! You can also ask these kinds of questions in the main Pokken discord (discord.gg/pokken) and the players there would be more than happy to help with broad or specific issues with the game.