Okay, so let's limit ourselves to the question "what would my N top optional rules be, for a S&Sian game of D&D" where N ≤ 6, since that is the number of optional rules I counted in Xoth's player's guide.
First, I would totally shuck out the existing options. Sorry! But in summary, 5E has already fixed everything you needed to houserule in d20!
The Massive Damage and No Alignment options has been previously discussed; Natural healing (since 5E already is more generous!), Ability damage (which 5E mostly don't have, and already have generous rules for when it does)
As for Treasure be spent, honestly even xp for gold is too fiddly (just replace with three levels of wealth*: Destitute, Scraping By, and Fabulously Wealthy and say you lose one step of Wealth automatically after each adventure).
Since 5E already works perfectly fine without magic swords, you could and probably should excise the section discussing that too, since it is framed as it being the problem it was in Pathfinder and 3E. Taking away the toys used to be a HUGE no no in D&D, but that was before 5E!
When it comes to magic, I would consider the benefits of telling players what they can do instead of what they can't. That is: to list the spells that you can cast instead of those you can't. The section is already so long it might be more user-friendly to just make a list. I would make this list as short as possible, and just say any other spell CAN be cast if you come across it (=GMs approval).
Here I would definitely not reflexively remove Fireball et al. After all offensive and damage is not a big balance problem (from the pov of the GM and adventure writer), and it IS supposed to be D&D after all. In other words: remove a casters ability to take his share of the spotlight and you could just as well prohibit the class entirely. No, I would keep most of the spells that "simply deal damage". What really is the dealbreaker is defensive spells - but 5E has already fixed the LFQW and buffing cheese! So I would humbly submit that most spells that need removin' are of the Rope Trick and Dimension Door and Create Food and Water variety, those that change or trivialize the environment.
Think of it like it was a MMO (yikes!): any spell that can't meaningfully affect the surroundings, that doesn't have a lasting effect or don't let the party trivialize said environment's restrictions can probably be allowed. yes it is true Conan never cast any fireballs, but a group prioritizing that simply never plays a D&D Wizard. In the larger context, the story does not change just because the Raiders died to fire instead of steel. So I'd leave the classes' ability to inflict damage mostly intact, or I'd prohibit them entirely. Otherwise, you're just creating trap options (one class choice being more nerfed than another).
Then I would choose my optional rules carefully. What are the most fundamental things D&D needs variant rules for (except spell selection)? Here's my shortlist of themes that could warrant a rules discussion - in no particular order:
- I truly believe it is much more fruitful to tweak the fundamentals of magic than to argue over which classes should be banned or reskinned

- no PC full casters: automatically makes every hero rely at least somewhat on his sword in a way that actually retains balance (every suggestion to make magic less safe or more controversial just nerfs caster classes)
- gender differences (is all I'm sayin...)
- if you don't want to prescribe a spell list, at least add a mechanism to use "heroic resilience" to "break" magic. It would go a long way of mitigating the worst of the "non S&S spells" without the complexity. I'd use Strength for spells with physical manifestations and Charisma for those without.
That's... basically it. I reread the thread but can't come up with any other. You'd still need to reskin certain classes, but that's just part of your "Xoth gazetteer" that already includes races etc.
*) I definitely think 5E has a weakness in "gold being worthless" for any campaign not focused on downtime (you can easily find lots of discussion on this issue). But honestly, S&S is not the right hill to die on - S&S isn't about wealth accumulation or building permanent structures. If anything, it is the direct opposite: why not wine and wench it all away since you don't know if you live tomorrow! So instead of trying to shore up the weakness here, I suggest you simply double down on the 5E characteristic of gold being worthless by not even tracking it in any detail.