In the 8-ball league that I ran for ~10 years, we had two categories of Most Improved. One was restricted to players in their 2nd session (since it's easier to show improvement with only one other session under your belt). The other was open to all other players. Our system was based on winning percentage improvement over the player's baseline.
We did not handicap our league... except for on a team basis. Each team had four players... an A, a B, a C and a D player... based on their cumulative winning percentage in our league. A new player to the league was assigned an initial rating after playing a number of games with existing members and the existing members discuss and agree on the new person's starting rating (A/B/C/D).
The teams were formed by blind draw. The only exception was if spouses or siblings, or close friends, wanted to play together, the total league voted on those proposals and a simple majority ruled. The league never allowed two A players to play together on the same team.
Each night of league play, each player shoots one game of 8-ball against the A, the B, the C and the D player from the other team.
Players who are D players one session can become a C player the next session if they shoot well... and vice versa.
Since we did not handicap individual players there was no sandbagging in our league.
10 years is pretty impressive. If I wore a hat I would take it off to you.
I knew there had to be LO's that had most improved player awards. I wonder if any of them used this to curb the sandbagging. Obviously, this wasn't a problem in your league.