You're actually hitting a real wall, but it's not a hard cap on potion strength - it's that you're comparing apples to oranges. Vendor potions and dungeon loot scale with your level, not your alchemy skill. Your crafted potions scale with alchemy skill, which maxes out at 100, so yeah, around 40-50 you start feeling the gap.
The thing is, you need the right perk path to keep up. The basic potion strength perks (the ones that boost effectiveness by %, stacking with each rank) help, but the real game-changer is the perk that lets you make potions with multiple effects - that's where your damage output actually lives. A two-effect poison hits way harder than a single-effect one at the same alchemy level. Make sure you're grabbing that one.
Also - and this matters - are you using the best ingredients for what you're trying to make? Like, if you're making damage potions with random stuff, of course they feel weak. Check what effects you actually need, find ingredients that have those effects together, and craft with purpose. A well-made poison with the right ingredient combo will punch above its weight even at alchemy 50.
You're not supposed to abandon it. Alchemy stays useful the whole game if you're actually using it strategically - poisons especially trivialize tough fights. But yeah, if you're just mass-crafting random potions and expecting them to compete with legendary loot, that's not how it works.