My ER9 brake calipers are solid. Instead of doing a full bleed, for regular maintenance I always do the top-off method by placing a funnel on the hydraulic screw port, fill it with mineral oil and then pump the brakes to remove air bubbles. I repeat the process on the other brake/shift lever.
I run the V1 ER9 calipers, the V2/V3 EGR calipers and a pair of GRX calipers with Ultegra levers on another gravel bike. I do the same method on all bikes. No issues. It's nice to have mineral oil compatibility across all platforms. This is the reason I'll never ride SRAM and their DOT brake fluid nonsense.