Couple of things to consider. Craft stones are nice and maintenance free. Buying from JB is much cheaper, especially if you have car to self-collect. I did this the last time, whole 20 boxes of it. However the labor to lay them is costly.
Since you opt out the kitchen wall, you have limited storage space, due to one less wall for top hung and bottom cabinet.
You can do an open concept kitchen, with a small bar counter with tall stools but kinda defeats the purpose as right in front is likely where you are gonna place the dining table. Also, open concept make air conditioning the living room a challenge.
Depending where you place and size of your fridge, likely you will have limited table top space. Again since you opt for no wall, imagine you have to place the fridge, sink and stove along the 3675mm kitchen wall, leaving you will almost no top at all.
You can consider hacking away the wall dividing the kitchen and service yard. I find such wall pointless and useless. Then you will have a bigger kitchen, looks more spacious and airy. However since yours is a new unit, hacking may require permit or totally disallow. Check with your ID.
Combining BR1&3 may prove to be disadvantage in the long run. Especially when selling, most buyers prefer more rooms than space in rooms.
Also since you had opt for flooring, ID may have problem finding similar flooring to replace the hacked walls. Looking at the floor plan, there is a pillar protruding out, which cannot be hack.
Overall cost is much higher, as you need to make good the wall after hacking, relocate the door to the combine rooms. I remember door frame are not cheap.
If you hack, you have to consider about air conditioning, a normal system 3 may not be sufficed to cool such a big area. I would rather to have BR3 as a gaming/entertainment room, with wardrobe built in.
This way, any late night gaming will not interfere with you sleeping in BR1. Kids will grow up, eldest already 5 years old, in time will wanna have own room.
Eldest will be 10 years old when MOP, if no intention to move again, BR3 can be revamp to be eldest room, leaving another room for the younger one.
Just my two cents. Have a good reno journey.