I absolutely recommend that you bring your TA up and your pH down after MLF is complete. This is best accomplished by tartaric acid, because wine bacteria will not consume tartaric acid; what you put in your wine will stay in your wine. This will certainly help with aging and color stability because both of these things are compromised by wines that have too high of a pH.
I am a big advocate of bench trials before you do a major adjustment to your wine like this. This will help you get the kind of numbers you need (probably at least a 1 g/L addition) without compromising the flavor. Or at least you will reach a “flavor compromise.” My ideal red wine pH post-MLF is around 3.50-3.75 depending on wine style goals and if you add too much acid your wine will taste tart with pronounced tannins.
Regarding potassium sorbate as a wine adjunct, I have to tell you I am not really a fan of it and rarely use it in my winemaking products. In fact, I only ever contemplated using it commercially once in a high-sugar, low-alcohol wine where I was worried about a yeast re-fermentation and wanted an extra level of security. However, I was able to achieve a 0.45 micron sterile filtration and had enough confidence to go to bottle without adding sorbate.
Potassium sorbate can indeed have an off-flavor and off-aroma. You should also be aware that it is only effective against yeast and not bacteria so it is of no use in retarding malolactic bacteria if you had residual malic acid remaining. Even if you have residual sugar in a fermentation, I am still a fan of sterile filtration instead of using potassium sorbate. I like to keep my wine as flaw-free and as intervention-free as possible and even small levels of potassium sorbate might throw something off balance. At the very least, tartaric acid is a naturally occurring grape acid, so if you adjust your pH with it, you are simply adding a little more of something that was already there to begin with.
