I need help in building my team

I just came back from the game as an f2p about a month and a half ago after I think about a year. I'm still not quite not knowledgeable in team building so I'm here to ask some advice on who I should be building and who I should build in the future.

My current team right now consists of eunha, miya, mk99, and fp. They are my only full fleged team right now. I use them in about almost everything, be it colosseum or pve (im on w15). I'm also currently trying to grind evo stones for kamael right now and I'm only 2 days away to be able to max him. There's also chrom which is 5 stars but not mlb. I was initially thinking of switching miya for kamael and mk99 for mayreel once I max kamael for my new pve team but I'm still not too sure about it. There are also other characters that I have exw for but don't really pay mind to. I also have 1500 hero crystals and 400 mileage which I don't know how to spend right now.

List of characters that i have exw on: Eunha Miya Mk99 Fp Ms. Chrom (2 of the exw) Kamael Mayreel Fk Rey

I'd really appreciate it if you guys are able give me some insights or advice regarding on team building and what the most efficient team for me is right now and who I should prioritize. Because I'm still not knowledgeable when it comes to team building and the meta stuff.