Good plan! I'll do what I can to assist. I'll try and break down my suggestions into segments so it's easier to read. Just a disclaimer, while I have beaten all the Souls games, I'm not an expert of optimising builds or min-maxing or anything. All I can provide is tips based on my own experience and knowledge.
Character Build - Stats
Let's take a look at the stats one by one and see how/if they're useful to you. I don't know exactly how you're playing, but I'll make a guess based on your equipment: melee tank with a touch of pyromancy.
Useful | Some Use | Not Useful
I've seen debates about the usefulness of health, since if you're blocking/dodging well it doesn't make a huge amount of difference. However, I definitely advise you to push it higher (to at least 20-25), depending on your needs. Why? You will die less, particularly if you're also wearing heavy armour like Havel's. Once you've pushed other useful stats as high as you want, make sure to invest plenty more here (maybe to between 30 - 35; see how you go).
Additional Spell Slots
How often do you use pyromancy? A lot? Occasionally? Never? If you answered anything other than "a lot," then don't bother going any higher. How many of your equipped spells do you use every single time you go out from a bonfire? If the answer is less than the current number of slots you have, then definitely don't put this any higher. If you want one additional spell, put one extra point in it since 16 will give it to you, but stop there unless you're using Pyromancy constantly.
For example, in DS3, I'm playing a pure sorcerer. Even then, I only have 4 slots and am unlikely to get many (maybe any) more, since there's only about 3-4 I use really regularly. I can always go back to a bonfire and re-attune if there's specific utility I need.
More Stamina, More Equip Load
35 is already quite high, and you could push it up to 40 if you wanted (I personally wouldn't), but do not go higher as that's where it caps.
Increases damage on STR-scaling weapons
Very useful, as you're already aware. 43 is excellent, and it's probably not necessary to go much higher unless you want to. 50 is the final soft cap, so definitely don't go past that.
Faster casting, more damage on DEX weapons.
Unless you find a STR-based weapon you want to use that also has DEX requirements, I wouldn't bother pushing this one any higher for your build.
Don't bother pushing any higher. First soft cap is 15 and the stat won't be that useful anyway.
Sorcery DMG and INT weapon DMG up.
Useless to you.
Miracle DMG and FTH weapon DMG up.
Useless to you.
Character Build - Equipment
I'd advise a change of weapon. The Dragon Tooth is very strong, but does not scale well and is slow to swing. Black Knight's Greatsword could be an excellent option if you happen to have it and still want a huge weapon. Otherwise there's Black Knight Sword which is a little smaller, but also a little faster. If you want to try something completely different, Butcher's Knife is a lot faster and should scale well with your high STR. Remember, speed of a weapon is important, especially when fighting a fast boss like Sif - the more time you have between swings, the more openings your enemy has.
Nothing wrong with your shield choice at all, though Divine is an odd enchantment as it doesn't give you personally any benefit. Silver Knight Shield would be a useful upgrade if you can grab it. Pretty sure that's the one I used, though there's los of other good ones. 100 physical block is your friend. If you want to upgrade a shield, just go the basic path with titanite shards.
Upgrade, upgrade, upgrade. That's the only way to make your pyromancy spells stronger. That said, if you don't really use them, don't waste the upgrade materials. Ignore the fact the flame tells you it scales with INT - that will not increase your spell damage. More info here.
It's a common mistake to assume that more physical defence will help you (well, sometimes it does, but often it doesn't when the cost is movement speed). Havel provides a huge amount of it, but if you're still dying then it might be worth exploring other avenues. How slowly do you move? If you feel like it's too slow, pick a different armour set. Black Iron looks like a solid option, and you should have it/be able to get it where you're up to. Chances are you'll notice you run/move faster and may even be able to dodge which, even with a good shield, is still handy. If you want, try something even lighter.
Oh, and don't forget you can mix and match armour from different sets to get to a weight/defense that works best for you. Nothing wrong with a couple of light armour pieces and a couple of heavy ones.
Ditch the Covetous Gold ring - it has use if you're farming items, but I wouldn't advise it for general gameplay. Absolutely go get the Cloranthy Ring and equip it ASAP, if you don't have it already. Stamina regen is good. Wolf Ring may also be useful, but stick with Steel Protection if you don't have it.
Here's pretty much everything you need to know about Sif and his attacks. There's also some decent strategy ideas there for different builds. I don't remember the fight well enough to provide much else useful on top of that, but I will impart some advice I've found helpful across all the Souls games:
Don't be afraid to change your equipment; different bosses call for different strategies. If a boss casts spells, make sure you're wearing armour with high magic defense, even if physical defense is lowered. Take a look at your rings and see if you can make a change that will help too.
I always hate changing my gear, but it can make a huge difference depending on the boss. Also, watching videos of someone else with your build-style winning the fight can be enlightening too. This one seems decent. Not perfect, but gives you an idea of how to dodge/where to stand.