How do I get better at complex grid-tied solar systems?

You can handle most solar systems now, and it is the complex grid-tied ones that still stop you. That is a good sign, not a bad one. It means the basics have become second nature and you have reached the genuinely harder work, where the interconnection, the codes, and the utility requirements pile up. Complexity like this is learned in layers, not in one leap. Each grid-tied job teaches you a piece, an inverter setup, a protection scheme, a permitting quirk, and those pieces accumulate into judgment. No one masters these from a manual alone. They master them by doing a few under the eye of someone who has done many, and asking why at each step. Find one experienced installer who does the complex grid-tied work and ask to assist on a job, watching closely for the parts that stump you. Then do the next one a little more yourself. You close this gap the way everyone does, one real system at a time, with a good teacher nearby.