What is the best/most fun way to play this game, for people who have come far or already finished it? I started out as the Samurai, going mainly into Dex, but so far I feel very outmatched against anything but a few easy enemies. Bosses are hopeless (mini bosses in caves etc have been fine though), and I have nothing for crowd control. Do you HAVE TO use magic for this game to really work?
Also haven't really found any weapons worth a damn, still using the Uchigatana.
Don't think in terms of "optimal." Every weapon is viable and, when upgraded, are more than capable of carrying you through the end game.
Instead try to decide what kind of play style you enjoy. For me, I enjoy two handing a small sword and going shieldless most of the time, hyper aggressive. But for you maybe you enjoy the stagger of a larger weapon or the delicate tap dance of creating space with a casting character. The first two areas are largely balanced to allow for experimentation. Try everything and only level your VIG and END for now along with enough points to allow you to wield any weapons you're curious about.
Once you decide what kind of play style you enjoy, then you can start to specialize in STR, DEX, both, or the various magics which is usually a combo of MND/INT or MND/FTH. As you specialize, you'll be on the lookout for rewards (namely talismans or weapons or spells) which fit your playstyle and which benefit your stat allocations.
The whole process of creating a build is very fluid and ultimately it comes down to tailoring your weapons and stats to your desired style of play. So long as you don't spread your points across too many categories there aren't really any "bad builds."
By the time you beat the second mainline boss you should have a grip on what youre shooting for and you also unlock the option to respec your character. This option is better suited for small refinements rather than sweeping changes (as you've probably started to make significant weapon upgrades) but it's definitely a tool you should use once you've settled on a general build archetype.