Make it fast. Performance guides covering Core Web Vitals, rendering, caching, and profiling to find what is actually slow instead of guessing.
Most database advice stays abstract because the concrete parts require reading a query plan. This goes into the specifics: schema decisions you can't reverse, index strategy, and how to find what's actually slow.
Adding a cache takes an afternoon. Choosing the right layer, invalidating correctly, and surviving a stampede is the actual work, and it's where most of the interesting failures live.
Server-side and client-side rendering are the two ends of a spectrum, and most of the interesting options sit between them. The choice depends on the route, not the application.
Mobile stopped being a special case a decade ago. The hard parts now are viewport units that lie, forms that fight the keyboard, and devices with CPUs unlike yours.
Most SEO advice aimed at developers is really content strategy in disguise. These are the parts that live in your code, starting with the rendering decision that nobody frames as an SEO decision.
Most image optimisation advice stops at compression, which your build tool already handles. The decisions that actually move your LCP are elsewhere.