Yeah if you're looking for a clan to ̶e̶x̶c̶e̶l̶ grow in with the aim of eventually becoming a competitively-viable strategic player, there's a few things to look for:
1. Does this clan have active events where you can play on strategic settings against players who are at least slightly above your skill level?
2. Does this clan have guides or other resources on the forum or elsewhere (or just knowledge of external resources they can point you to) that will help you learn the game mechanics and tactics?
3. Does this clan have mentors or invested players who are available and willing to play with you, advise you, and train you?
4. Does this clan have game analysis going on somewhere? It might be in a forum thread where people post games, in chat, on Discord, anywhere. But a clan that regularly analyzes games or at least discusses winning/losing tactics and strategies in finished games has a good chance of being a healthy strategic environment that helps you learn from concrete results.
I have not been in Excel so I cannot attest to the answers, but these are the four questions I suggest newbies ask themselves before joining a clan like Excel that promises a growth environment. Another proxy you can look at is whether the clan has any players that rapidly improved recently (for examples of rapidly improved players, look for players with good ladder ratings and high recent winrates that indicate they were/are underrated; see Shredtail in MASTER from 2017-2020 - http://md-ladder.cloudapp.net/player?playerId=4018123716
, _ in 101st from 2017-2020 - https://www.warzone.com/LadderTeam?LadderTeamID=342
, Ursus in TSFH during 2020 - http://md-ladder.cloudapp.net/player?playerId=3367992188
, and more modestly Gus in Discovery in 2021 - https://www.warzone.com/LadderTeam?LadderTeamID=26835
My expectation based on chance encounters with Excel players and asking them about the clan is that Excel meets criterion 1 narrowly (coin tourneys on a mix of strategic and unstrategic templates) but fails 2, 3, and 4.
Best of luck finding a clan to grow in! Be careful as you search for a growth/training clan because almost every clan with strategic ambition will advertise itself that way (to get members) but most do not actually make the cut.
Edited 4/1/2021 19:56:06