Frankly, I was a tadbit disappointed.

Alternative phrasing, sounds a bit more natural to me.

Despite the average proficiency level there being complacentlyrather high, the way the whole event had beenwas organized was rather underwhelming.

Was sounds slightly better here since the organization was, presumably, still relevant/on-going

The main problem was that there were so many participants and so littlefew opportunityies to speak out.

Speak out (about something) is normally used for protesting in my dialect.

Everyone wanted to add their two cents, creating more like a queue to hold a monologue than a space for constructive book discussion.

Just a note that queue is British English, in American English we use line.

The main problem was that there were so many participants and so littlefew opportunityies to speak out.

I think "little" would slide when speaking but it does feel a little off. "Little" sounds right with "opportunity" though. ie. "Even having been given so little opportunity, he prevailed." Gee, I can't really explain what's going on here...

Frankly, I was a tad disappointed.

Personally I would say "a tad bit", but this is also fine.

Despite the average proficiency level there being complacently high, the way the whole event had been organized was rather underwhelming.

I've always used the word "complacent" to mean unconcerned, uncritical, or oblivious. For instance, "to be complacent about systemic injustice". However, some dictionaries list definitions which imply smugness, and although a complacent person can be smug, I've never used this word like that. I guess some people use this word differently though? Whatever the case, it sounds odd to me. Complacency implies laziness and a lack of knowledge, so having a "complacently high proficiency level" feels paradoxical to me. Although, maybe "complacently knowledgeable" could be an interesting way of saying one has knowledge without wisdom. I don't know.

If you want to imply that these people were really smug, I might say something like "pompously".

The main problem was that there were so many participants and so little opportunity to speak out.

Everyone wanted to add their two cents, creating more likeof a queue to hold a monologue than a space for constructive book discussion.