Sola Gratia was the first complete album I listened to and it didn't really do much for me. I listed to the newest Transatlantic album, both Forevermore and Breath of Life but those felt long-winded to me. I do plan on going back to listen to the The Absolute Universe again because it was a lot to take in on one listen but I'm not sure if I'll enjoy it as much as I&D.
Glad to hear you're getting interested in Neal's music! Since you liked I&D and didn't care as much for
Sola Gratia, it might be worth pointing out the distinction between Neal Morse solo and The Neal Morse Band, in case you weren't already aware of it.
Neal Morse solo is all music written by Neal and with him doing most of the guitar, keyboard and vocals, featuring Mike Portnoy and Randy George playing drums and bass.
The Neal Morse Band is written more collaboratively by Neal, Mike, Randy, Eric Gillette (guitar) and Bill Hubauer (keys), with vocals split between Neal, Eric, Bill and Mike in approximately the proportions that you heard on I&D. I think it's fair to say that a lot of people, myself included, feel that there's an added creative energy to the NMB releases over and above the Neal solo releases. The other NMB albums are:
The Grand Experiment,
The Similitude of a Dream and
The Great Adventure. These aren't in exactly the same style as I&D, but you're probably more likely to enjoy them than other NM releases based on what you've liked so far.