Well first of all, you're going to want to put points into endurance anyway. It's considered the king of stats. It increases equip-load which isn't just useful for heavier weapons, but it's also useful for wearing stronger Armour (therefore you effectively get an hp increase). Plus, you're going to want to increase stamina at some point.
As for using heavy weapons, you are missing two key details. The first is poise damage. You know how poison builds up when those rats attack you, but you only get poisoned when the bar is full? Well the same thing applies to hitstun. Every weapon in the game does a certain amount of poise damage which slowly fills up an invisible bar every time an enemy is hit by it. Once the bar is full, the enemy stops whatever it is doing (including attacking) and is momentarily staggered as the bar resets. Heavier weapons deal more poise damage and lighter weapons deal less. Smaller enemies are staggered easily, while bigger enemies like black knights and bosses usually require a few hits from big weapons to get interrupted. To compensate, bigger foes are usually stunned for longer once their defenses finally do crumble allowing you to momentarily wail on them without fear of retaliation. There really is nothing more satisfying than charging at a boss who's winding up a big attack, while knowing that it won't hurt you because your next hit is the last one needed to break its poise.
The second thing you're missing is the fact that UGS's are usually late game weapons. You get a few big weapons early on like the GreatAxe and the Greatclub, but those are shit and they have terrible movesets (I've never used the Zwei, but I know the community loves it for some reason). I was mostly a halberd dude until I found the Black Knight Sword (not the GreatSword). That thing was a beast that owned even without any titanite. So keep going, you may find something you like eventually.
Finally, I want to mention the fact that Strength weapons are meant to be used two-handed. In addition to improving the move-set, wielding a weapon with two-hands increases your effective strength by 50% which grants them a decent damage boost compared to their one-handed dexterity-based counterparts. I noticed you complain about strength weapons doing underwhelming damage which to me is a sign that something went wrong. The whole trade-off with them is that you deal more damage (both per hit and per second) at the cost of being less flexible than you would be with dexterity weapons. It's easy to underestimate attribute scaling, since you only see it when you wield the weapon, but weapons with larger base damage benefit from scaling more. They also get a higher damage bonus from titanite.
you can seemingly deal just as much damage by enchanting a dagger or shortsword and just going to town with it. Why would anyone use an ultra greatsword, even just in PvE?
If you meant enchant as in temporarily buffing your weapon with spells like "magic weapon", "lightning blade" or "greater magic weapon", then you do have a point there. The damage increase from those spells scales solely with your Spell Power (a function of your wand and your INT) without taking weapon base damage into account, making it optimal to use those spells on fast weapons. If you're casting those spells though, then you're playing a wizard and INT is your primary attacking attribute. You have no reason to level STR in that case, and thus no reason to use bigger weapons.
Finally, let me finish with a basic guide to DS1 levelling.
1. Never level up
Resistance, (trust me on this one. It's a terribly designed stat)
2. Leveling is usually about choosing between
HP,
Endurance, and your
primary attribute. If you use weapons that scale best with strength, then your primary attribute is STR. If dexterity, then it's DEX. If you're a wizard, then it's INT. If you cast miracles, then it's Faith. You can rarely go wrong if you consistently make a choice from one of those 3.
I hope you have fun.