A TRACON Controller reported an aircraft descended below its assigned altitude and flew below the Minimum Vectoring Altitude.

Narrative

I was vectoring Aircraft X for the ILS XXR at ZZZ and told the aircraft to descend to 65 (which was the lowest MVA altitude for their position) in an attempt to get them low enough to intercept the ILS. I then diverted my attention to some other traffic and came back shortly after to issue the approach clearance. As I was issuing the approach clearance for the ILS XXR into ZZZ; I realized Aircraft X was already at 60 but should have been at 65. The pilot did not acknowledge the approach clearance right away and kept descending to 56 in the 60 MVA before climbing back up. I told the pilot again to maintain 60 but didnt get a low altitude alert out. I called ZZZ tower and also told the supervisor on duty. The pilot never called back so I didnt get a chance to talk to them. After listening to the audio the pilot read back 55 and not 65; I did not catch the bad read back.Technique wise I like to get an aircraft as low as possible for the ILS XXR into ZZZ (65; 62; etc.) because it has a steep approach path. I think from here on out I am going to stay with normal IFR altitudes (60; 70; etc.). I think introducing a non standard IFR altitude assignment with a 5 or 2 just introduces too much risk of a bad read back in that area. I think it is better to risk a go around because they were too high for the approach. Also; my family has been dealing with the flu the last couple of weeks and I have been extra tired. I already missed one other read back during the session and probably should have asked for a handoff after that. In the future if I am feeling that fatigued I will definitely ask for some help.