I would take your time in this process. All of your pre planning will save time which saves lots of money. Also, I wouldn’t be so concerned with adding some sq ft if that is what you want. Cost per sq ft goes down as you build bigger, to an extent. All of your fixed costs remain the same no matter how big you build: power, water, septic, etc
Windows and doors cost money but a few extra studs and an extra sheet of drywall is extremely cheap to make a cramped house feel much more spacious. Lumber is extremely cheap right now.
However, you’re at nearly 1800 sq ft for a 3/2 which is honesty a good size.
I’ve done this a few times for myself and countless other times for clients, and I’m still drawing up the most efficient plans, but I’m getting close.