You are in no way obliged to use the SCA agency agreement.
But what is the alternatives?
You may change the SCA agreement by striking out or adding clauses with agreement of the strata manager. But it’s unlikely that a strata manager will agree. They will give lots of reasons, both genuine and bogus not to. Remember that the SCA agreement is written by the organisation that strata managers are members of and the agreement favours the strata manager.
Write your own agreement. Of course you can’t take the SCA agreement as a basis because that [document] is copyrighted. You would need to hire a lawyer to draft such an agreement, the cost of which would be thousands.
Perhaps , if JT reads t his, it’s a call to get OCN to draft a contract that committees can put to their strata managers, or perrhaps the government could draft a contract similar to what they have done for residential tenancies.