I'm confused while organizing characters with mid-high stats (like Byakuren, she has versatile stats, but I don't know what to do) and also Support. :|
It's based on how you want to balance your team, with healers, buffers, debuffers, tanks, and attackers. For one thing, you're definitely going to need a pure tank of some sort or multiple bulky characters to compensate for the lack of a tank. Some would argue that you need more bulk than that, but that's really up to the player's play style.
But following this, if the need for bulk, survivability, and buffing is already satisfied, then someone like Byakuren should probably be built like ZXNova's Byakuren (Bulky Offensive Support). This would let her deal damage as well as apply the myriad of debuffs and statuses that her offensive spells come with.
In case anyone's wondering, (I highly doubt people can't figure this out if they were actually wondering but anyway) in terms of survivability:
Tank > Tanky >= Bulky > Frail / Glass Cannon
(Tanky is greater than or equal to Bulky, since I'm sure some people use the terms interchangeably to mean the same thing).
I really have trouble describing my thought process on the fly, unless asked about a specific aspect of it, since most of this team stuff building comes naturally to me. I'll never say that my team or any team for that matter is the best, since it also largely depends on the player's play style.
There are some instances where a player's play style makes the game more difficult than it needs to be (like Ghaleon's policy of not swapping out his chosen 12 just for a specific boss fight) but that's their choice since they get more enjoyment out of the game for whatever reason (like getting more challenge for example). I myself don't like switching up my characters too much after deciding on a team of 12, unless I felt like a different character can accomplish the role I wanted better.
An example in my case would be me swapping out Satori, which I loved dearly for her spellcard copying, for Renko because at the point of the game I was in, Satori was not able to deal damage to a satisfactory amount. She was mostly just a second source of Reimu's Hakurei Barrier / Exorcising Border without the defensive stats Reimu had. Renko's buff however, was just as strong as Reimu's Hakurei Barrier defensively but it also buffed both ATK and MAG. This meant I could leave the group buffing to Renko while saving Reimu just for the group heal, which means I don't have to risk losing two characters when things get hairy and I need to heal.
I started with 5 Support / Tank characters and 7 Attacking characters and slowly transitioned into the team I have now which consists of 8 Support / Tank characters and 4 Attacking characters. This was due to the fact that I felt like I needed more and more survivability as I neared end game, so I added some bulkier characters / rearranged some from an attacking role to a supporting role as I went on.
And just asking, is it possible for us to complete level of all skills for all characters? (Except if the character learn Tomes for other skills)
Yes, you can max all skills for all characters. It just depends on the characters skill set as for when you can max everything. Some characters have more costly skills / more skills than others so the level at which they can have max everything differs. For example, Alice needs to be level 178 to max all her base skills without the use of Training Manuals.
(She was the one that was the easiest for me to count up since I basically maxed everything I wanted on her base skills. This is why I gave her Transcendent subclass [which needs 46 points to max] to make use of her excess skill points that I wasn't using.)
This is obviously quite far into the game so before you reach that point, you should be prioritizing the skills the mean the most to you. Outside of support characters, I generally never level up the character's spells until I've gotten all the passives that I want. I know that leveling up most attacking skills would increase damage output, but I don't know by how much, so I can't reasonably weigh how much leveling up that attacking skill is worth compared to leveling up a passive.
On a separate note, I like how two simple questions has me going off on tangents about various topics, some of which aren't even necessary in answering the OP's question.
Also on a separate note, I think the Palace of Earth Spirits Synergy group is highly awkward. None of the characters in the group are really suited to be built for staying power, so its hard trying to make use of their synergy passive.
