Seeking advice for a development site for sale (Brisbane City rezoning area)

I am now one of several owners in multiple joint development land for sale in Brisbane Northern Corridor plan, which has been rezoned to higher density by Brisbane City Council.
Total size of our joint development site is now over 2000 sqm which I believe the max GFA is 2.5x and max BH is 6 stories and 21m.

Now we have currently one local sales agent working for us. (not sure about the history as I joined later)

One of the main sales person advised me followings;
1. they are mainly focusing on offers from small to mid sized developers because development site over 2000 sqm normally starts attracting "big" players/ developers who are in general not paying highest price so our agent wants to avoid them.
[Question] Is this comment true in general? and is this sales tactics rational?

2. Our site is for negotiation, not for auction or tender. I don't know the reason behind this decision.
[Question] Is this sales tactics rational? would you hire same sales method for this development site sale?

3. They have at least received an offer from a small sized developer with a condition of payment installment ie. partial upfront payment first then subsequent payments as their unit sales are progressing etc.
[Question]I thought the purchase transaction is completed with full payment on contract exchange date. I feel like this installment condition is risky. Is this offer condition normal and often used by developers? What's the risk?

4. As for this particular offer, one of our joint owners believed the offer price is too low so she didn't accept that offer.

5. Firstly I want to be 100% sure that our sales agent is only focusing to max our sales profit (and consequently they will get higher commission from this sale), nothing else to hide. I am not yet 100% convinced.
Therefore I start wondering about engaging another sales agent for our development land sale.
[Question] what pros and cons we need to be aware of in this option?

As this is my first time ever to be involved in development site sale, let alone investment property sale (although I owned a few IPs now), can anyone experienced in this field share your knowledge, feedback/comments and recommendation in my situation above?