Multiclass Paladin + Pact Magic = What paladin spells can I ...
The spell prevents them from regaining spell slots; that does not mean they cannot cast the same spell using a different spell slot.
Cleric Ritual Spells | Astral Reach Wiki | Fandom
And the Players Hand Book states that "You can a Cleric spell as a ritual if that spell has the ritual tag and you have the spell prepared".
Conan Exiles: Defeat Volcano Boss and Get Trapezohedron
While you have to be able to cast 7th level spells, the rules state that the spell cast using higher level spell slot assumes the level for that casting.