I'll address what you brought up about the rock you don't like first
When I decided to change some of my 'scape, I just got wet up to the armpits and removed what I didn't like. I transferred the corals to different sections of the tank, sold what corals didn't fit, and put the removed LR into the sump for starters
Then I slowly removed/sold the rock from the sump until it was all gone. This gave the DT LR time to keep up bacteria-wise
If you do really want to do a tank upgrade, and it were me;
I'd ideally like to setup a completely new system and slowly transfer everything, but that may not work for you, so you'll have a busy weekend doing a 'tank transfer'
As far as size goes;
Going up or down in size will depend on your personal thoughts/space, and time/money budgets
I never really thought about what my tank was costing me for quite some time. Then one day I added stuff up and am glad I only have my current setup. I couldn't dream of the time and money spent on say a 300g system right now, but that's me.
Think about what you like and don't about your current setup and what you'd change
Then the planning starts and you don't sleep for days while you think about it
And what you stated is about it, if you have the space ...
"set up the new tank with new sand and rock etc and let it cycle and leave livestock in the old tank until said new tank is ready"