Flatpak Discord runs on Wayland on my machine without me needing to add any flags. Have you enabled the toggle for Wayland within Flatseal?
Also I believe UseOzonePlatform is no longer needed by Chromium-based browsers and Electron apps, --ozone-platform-hint=auto should be enough. --ozone-platform-hint=auto --enable-features=VaapiVideoDecodeLinuxGL,WebRTCPipeWireCapturer enables Wayland and HW acc for me on Chromium, Slack, and Webcord, for example.