Steam's prices are inclusive of any sales tax (if it says 4.99 you're going to pay 4.99)
This is not true (where it matters). They say so on their faq.
https://partner.steamgames.com/doc/finance/taxfaq#CurrentRates
Prices for Canada and US are always listed without tax. Same as Itch, as they are in the US. Where applicable, tax will be collected. And on top, Steam has regional pricing, often "undercutting" itself.
Also, you can give out a Steam key with purchases made here. There even is a mechanism for that, and it will read on the bottom of the page like this: "Your purchase comes with a Steam key"
Some games on that list have that mechanism. Try to compare prices. Be sure to look at the prices on Steam with a vpn locating you in the US, or you will see not the correct prices you would have to compare. And to be clear, you are selling on Itch, an US based store, so you would have to compare to the regional pricing in the US from Steam.
https://itch.io/games/top-sellers/steam-key/store
If you need to use Steam features, of course, people do need the Steam version to use those, But see above, you do can sell Steam versions here, no problems, no heachaches for pricing. Set both to $ 5 or whatever, it is both shown without tax.
(It might be more complicated, if Steam requires you to name a price for your own region and they thus would calculate the US price backward. But if they allow you to name the US price there is no argument about any tax issues in pricing and undercutting.)
You might have to put some disclaimers on itch, to make people aware that your game is Early Access though.