In the Jellyfin app on your phone, click on Settings > Playback (not the dashboard) and set the max transcode resolution to 1080P. Right now the output of the transcode is 4K.
Jellyfin 10.10.7 (Docker)
Ubuntu 24.04.2 LTS w/HWE
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
4x WD Red Pro 6TB CMR in RAIDZ1