Correcting seasoning is as much art as it is salt. Even though salt is usually what is reached for first.
Acid is one of my favorite choices. One because it doesn't add appreciable sodium; Two it tends to make things taste fresher and brighter. Fresh lemon juice is the most versatile here. Then Cider Vinegar and Rice vinegar. Cider vinegar, though fermented still packs fresh fruity tones. Rice vinegar can be vaguely citrusy but milder. This makes them versativel compared to wine vinegars that I find more limited in application for correcting seasoning.
Hot sauce, but mild hot sauce. Hot sauce packs acidity as well, but includes the heat factor. Chile sauces trigger pain receptors which seems to sensitize the tastebuds to other flavors already present. Similarly, mustard, horseradish, wasabi and black pepper all trigger the pain receptors. The strong sinus kick of horseradish and wasabi make them difficult to use for correction, But dijon mustard or yellow mustard can kick up flavors too.
Herbs, espcially dried ones, usually benefit from some cooking time so you should be "correcting" along the way. Paul Prudhomme did this a lot with his spice blends. In his early books, he'd give you the list of ingredients to blend. In later editions he began to just list his commercial blends. Anyway, because herbs and spiced develop different flavors at different lengths of cooking and treatment, he'd instruct you to add some of the spice blend at various points throughout the cooking. He wanted the dish to take on as many different characteristics of the flavors as it could in the time it cooked. Similarly, you should be tasting your dishes along the way and seeing if the flavor is on track. This is a skill that takes practice to develop. It means tasting your food (in food safe ways) to ensure your seasoning arrives at the desired end. And learning how flavor changes with the cooking.
Special recommendation to furikake. Grind it up if you want it to disappear into the dish as a finshing accent.--watch the salt addition though.
Fresh herbs to consider for correcting seasoning. Basil, Cilantro (on appropriate dishes). Parsley, beyond the garnish factor has a peppery freshness I like.
Use the best salt you can. And i don't mean fleur de sel, though that's fine too. No, I mean use high salt seasonings to build flavor but without over salting the dish. This is your "bottled flavors" you decried in the first post. Or Anchovies.
Soy Sauce, Fish Sauce, Worcerstershire sauce are all high sodium seasonings with great power and versatility. They are bottled complexity. They are complex because they are fermented ingredients. Time and fermentation build myriad new chemicals and flavors from simple ingredients, Rather than a cheat, they are in the same category as wine, but so strong, you use them lightly. Rather than building flavor with long cooking in the pot, they build flavor over time in the ferment. It's why Asian food can cook quickly and be full flavored. Used lightly, they don't make your dishes taste Asian, just complex. And salty. So adjust your added salt accordingly.
Similarly, there are other fermented high salt products, most notably Parmigianno Regianno or other aged grating cheeses. These pack a lot of savory salty flavors. It's one of my favorite salty flavors. Similar flavors also appear in the best fish sauce.
A good stock base. Where restaurants build with demiglace, a home cook can bunch up a weak broth or sauce with some low sodium base of good quality. Better than Bouillion makes some good ones that are widely available. Better still are brands like More than Gourmet, but they are commensurately more expensive. Again, this usually benefits with some cooking time, so you need to be tasting as you cook to correct in time. But you can save your dish with the timely addition. Similar compounds again come from Asia--indeed, the granulated Chicken Base was known as Chicken MSG in the industry at one time as it was used as seasoning. But also Oigatsuo Tsusyu. This is essentially dashi and soy concentrate used for quick broths in Japan. It's not a bad soy substitute in it's own right if you want some fishiness for whatever reason.