You are correct to ask these questions!
First, for you to progress at the max . rate, you need to be comfortable.
So, switching to a board without a centerboard/daggerboard is certainly possible, but the amount
of true upwind sailing you need to do at the sailing sites you choose might be a deciding factor also.
The Rio M is going to be a bit tippy at first for a 115 kg. (253 lb.) sailor.
At your skill level, you need significantly more volume than 125 liters.
If you decide on a GO 155 (pretty good choice, but it gives you virtually no ability to stay upwind with your
current rig steering skills set) you will have a little trouble at first, figuring out how to "tip" your board to stay upwind in sub planing conditions.
What windspeeds do you think you will be sailing in?
Sail size will be pretty important in your initial success. Too big and you won't be able to handle it easily,
too small and you will hardly get moving.
So lots of choices and decisions to make.
Perhaps you might buy a Start M (210 liters x 94 cm wide) as this suits your current weight and skill set the best. Look for a 6.5 -7.5 m2 rig with a fairly light (min. 75% carbon) mast and a no cam sail.
You can always retract the centerboard and practice sailing the Start M as a shortboard and use foot steering vs your current rig steering.
The Rio M is a possibility, but at 195 liters x 80.5 cm wide, and you can do the same thing, with the same rig (s) but initially it will be a little tippy.
It is probably more important that you be able to use your current skill set and have success right away.
Perhaps buy a used Start M or Rio M and a used 7.5 free ride rig as a transition to more wind and getting used to sailing the board with the centerboard up (retracted) but with the option of put the CB back down if you get too far downwind.
Hope this helps,