Following your recent notice concerning the Science Exhibition, I am writing, first and foremost, to express my gratitude for considering our opinions regarding the event.

The commas can be optional.

In addition, I would like to inform you that, in my viewopinion, tickets should be free, so that a hularge number of people couldan attend, even if they could annot afford it.

As rRegarding the question of what activities can be performeddone, I must confess to being indecisive.

We usually use the "do" verb when describing activities.

Despite that, I consider it an fantastic opportunity to get our community interested in some environmental issues and theirpropose possible approaches.

Following your recent notice concerning the Sscience Eexhibition, I am writing, first and foremost, to express my gratitude for considering our opinions regarding the event.

In addition, I would like to inform you that in my view tickets should be free, so that a huge number ofmore people couldan attend, even if they could annot afford it.

"Huge" is too informal.
This is describing a future event so "can" is better.

As regards to the question of what activities can be performedthere should be, I must confess to being indecisive.

"Perform" would be more appropriate if the activities are performances.

It would be a good idea if people could take part in workshops related to the topic,; however, It may increase the budget.

"However" is a conjunctive adverb and should not be used as a conjunction.

Despite that, I consider it an fantastic opportunity to get our community interested in some environmental issues and their possible approachesolutions.

"Some" isn't necessary here. Use "certain" or "particular" if you want to specify that this isn't referring to environmental issues in general, for formality.

Thank you very much again for your interest in feedback from the community.

The original sentence sounds more like something that would be used if you were the event organizer responding to a letter.

In addition, I would like to inform you that in my view the tickets should be free, so that a huge number of people could attend, even if theyspecially those that could not normally afford it.

As rRegarding the question of what activities can be performed, I must confess to being indecisive.

Despite that, I consider it an fantastic opportunity to get our community interested in some environmental issues and their possible approachesolutions.