Ngl, I think store-bought cake mix often makes a more consistent layer cake than doing it completely from scratch.
Honestly, after my last homemade cake crumbled when I tried to stack it, I used a box mix as a backup and it was way more reliable for a tall, even cake, which probably makes me a baking heretic.
My from scratch chocolate cake for my dad's birthday last month was so dense it could have been a doorstop. I feel you on the crumbling thing, it's so frustrating when you put in all that work. Box mixes have those stabilizers and precise measurements that just work every time. I don't think it makes you a heretic, it makes you smart for wanting a cake that actually holds up. Plus, you can still add your own touches like extra vanilla or real butter to make it taste more homemade.
Actually, @iris940, I have to push back on the box mix idea. Those stabilizers you mention often give cakes a fake, processed flavor that extra vanilla can't hide. Mastering from-scratch means learning little tricks, like creaming your butter and sugar really well to add air. Once you get that right, the cake is moist and light, nothing like a doorstop.
Add a little extra oil or an egg yolk to your from scratch batter. It helps keep things moist and less crumbly. I do it all the time when I'm trying to stack layers.