Dammit, I'm an idiot for not realizing this earlier. Apologies to the OP for not seeing the trivial, obvious solution.
Level shifter: PiHut TXB0104
TXB0104 can definitely three-state the pins: it's got the OE pin for exactly this purpose. When it is low (OP has it tied high), all signal pins are in high-impedance state, as if they were disconnected.
However, TXB0104
datasheet notes that VCCA ≤ VCCB; VCCA = 1.2V to 3.6V, VCCB = 1.65V to 5.5V. OP has it the exact wrong way in the schematic, but since the acceptable voltages are silkscreened on the board, I assume they have them right.
The simplest option is to connect the CS1_5V pin (the active-low select for the ADS1220) ALSO to the TXB0104 OE. That way, whenever the ADS1220 is enabled (CS1_5V is LOW), the TXB0104 is disabled and all its pins in high impedance state ("disconnected"). No other changes are needed.
(While OE pin is powered by VCCA, it is safe for voltages up to 5.5V, so it is perfectly okay to use VCCA=3.3V, VCCB=5V, and connect OE to a 5V I/O pin. The 3.3V VCCA simply means that on the OE pin, LOW is below 1.2V, and HIGH is above 2.2V.)