> Yes but in practice they delegate this power to the executive.

No, they do not delegate the power to lay (set) taxes to the executive, they do assign the executive the function of collecting the taxes laid by Congress.

> Congress doesn’t run the IRS themselves after all

The IRS doesn't freely set taxes, it collects the taxes set by Congress.

The moment Congress authorizes that the Executive may use discretion then the Executive can effectively levy taxes. They may be wielding a bat owned by someone else, but who swings it is ultimately what's important.

Now I'm generally of the opinion that Congress shouldn't be allowed to give the Executive discretion but seems no one agrees with that and Congress would rather let the Executive write "not quite laws" on their behalf.