Alright so I have tried both CEX and DTL and both will not work on a PAL SCPH-90004 for ORIGINAL ps1 games, backups work sometimes but not always on DTL option. I have tried DTL-90000, DTL-900001, DTL-90004 and none of them read original NTSC/UC Final Fantasy IX unfortunately, so I think PAL Deckard consoles just don't have the option to run original PS1 games and backups are iffy as well.
If you patch as DTL-H90001, every Ps1 (originals and backups) game should boot.
If you patch to DTL-H90004, only PAL games (either originals or backups) will boot. You can boot backups, firstly patching them to europe region (with disk patcher 3.0).
If Ps1 games boot only sometimes, then most probably your CDs are ruined, or also your Ps2 lens (maybe the CD trimmer needs an adjustment).