Here's the thing. As long as you get a finite number of slots, you are going to have to find a way to cull mods as often as you gain them. An increase in mod slots is going to give you some weeks or months off, a vacation from culling mods, if you will. But then your inventory will be full again eventually unless and until you learn to sell as many mods as you keep.
That's just basic math.
So think about this problem you have. Your mod inventory is full. That's actually great news! It means under your old strategy, you were keeping more mods than you sold.
But NOW you have a bunch of mods that meet whatever criteria you used to use. Now you get to make more strict criteria, and every mod you save in the future will meet a higher minimum standard than the mods you've saved up to now.
Sure, some of your current mods exceed those standards. Possibly by a lot. No worries. You get to keep them. But the new ones will ALWAYS be better than the worst of your old ones. This is a new era for you. An era of better mods.
Embrace the dawn; welcome this new day of better mods.