Update! I finished Phantasy Star II last night.
The game had started to get old around the time you have to go to the four Dam dungeons, mainly because of the repetitive nature of the background and enemy types, as well as the high encounter rate and strength of the enemies. One thing I felt like the game kinda lacked was boss battles - in most JRPGs these are kinda where you get to show off all of your cool abilities, but this game only has two of them! (The "Army Eye" battle doesn't count since you can't win it.)
But anyway, the game picked back up for me on Dezoris, I actually quite enjoyed the Nei-item dungeons, the run-down villages, and the spooky abandoned spaceport with the stray cats and newspaper clippings.
I arrived at Dark Force with Rolf, Rudo, Anna, and Amy all at about level 26-27 and was soundly thrashed. I warped back to one of the Nei dungeons and did some grinding - I didn't really want to go all the way to level 35 (when Rolf learns the Megidd technique) since that sounded awful, so I decided to hit 30 and then see how I did.
For the second round I did much better. By level 29 Amy had learned GiSar which really helped, and I rearranged my party and equipment a bit for more effective battle. I managed to kill Dark Force but lost Anna and Amy, so I had to warp back to town to clone them before proceeding to the final-final boss.
Mother Brain wasn't nearly as difficult as Dark Force. I was able to manage her attack-all effects pretty easily with GiSar and Anna/Amy using Trimates as needed. I beat her with all party members intact.
The ending was a surprise for me! I guess I won't spoil it but it didn't feel like the ending of any other JRPG I've played.
So next is either finishing Shining in the Darkness or starting Phantasy Star III.