With Diouf, it would make sense to protect Oh Ji-young since you're in a "win now" mode. Without Diouf, I would rather think about a long-term rebuild and protect one of my OH kids like Ko Ui-jeong and/or Lee Sun-woo instead, on top of Park Eun-jin and Jeong Ho-young.
Let these kids grow together and build a damn dynasty! They have the potential for it. Lee So-young can mentor them and she has a good 5 or so years of dominant volleyball in her, provided she stay healthy.
For 2021, go with Noh Ran, Chae Seon-ah (converted) and draft a libero for your pipeline. Or trade a depth OH (Ko Min-ji for ex.) for a depth libero.
You can make do with an average libero in V-League, but you'll never go anywhere without a strong OH lineup. If you look at the final rankings, the results are pretty correlated with OH strength and balance. The foreign OPs basically cancel each other out as the disparity in level between them, at least this year, wasn't THAT high to make or break a team, compared to the disparity in OH strength.
S- KYK, Lee Jae-yeong
1- Lee So-young, Kang So-hwi
2- KYK, Kim Mi-yeon
3- Pyo Seung-ju, Kim Ju-hyang
4- Park Jeong-ah, Jeon Sae-yan/Moon Jeong-won
5- Choi Eun-ji, Ko Ui-jeong/Ji Min-kyung
6- Go Ye-rim, Hwang Min-kyung
Obviously, the correlation isn't perfect and which pair is better is debatable. Hyundai in a normal year should be higher, but they had a lot of off-court drama before the season and both GYR and HMK vastly underperformed. Park Jeong-ah is better than KMY, PSJ and KJH, but she can't receive and has to be paired at all times with an offensively challenged defensive left. They still barely missed the playoffs by a point or so...