Would instrumentation without fusion be a better alternative than fusion when growth potential is remaining?

Not Typically. The instrumentation is an internal splint to allow fusion to occur in a controlled fashion. Instrumentation without fusion will lead to either loosening or fracture of the hardware.
At age 23..... Have you tried interventional pain management 1st. Of course I do not know your back condition, but you are placing yourself (at a very young age) to ruin the rest of your life. At least get a second opinion.