You should try solo Q'ing now, you might seen bigger differences then before. I'd also like to point out that as you've said in your post, you gain 17 per win and 19 per loss. that's a -2 LP net loss. You've also said that solo Q wins give you 23 LP and losses lose you 20, that's a net increase in +3. That means that going 50/50 in solo Q would help more than 50/50 in Duo.
On my climb to plat, most of my games were solo Q because I knew it would help me the most, so at the end of my climb, when i reached plat and then plat 4 promos, I was gaining 28-30 LP per win and only losing 12-15 per loss. Then I played duo to help get my friend to plat, when we won I gained only around ~26 LP and a loss was ~-17 LP. It's from experience and what I remember a rioter saying is why I wrote that.
Sorry if that didn't help still, I tried.