> 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.

They don't delegate the policymaking. Tax code is always congressionally approved, and I'm unaware of any even remote argument that changing tax policy is delegated to the executive.

OTOH enforcement of congressional policies is basically always the role of the executive, so the fact that the IRS exists and does things doesn't really impact delegation.