The key is to start small. Practice keeping a simple roll going (TITM is a good choice because the "inside" thumb notes align with the two beats in each measure). Forward-backward is a good roll because of its symmetry. I would stay away from using a roll that "crosses measures" as you want the banjo roll to be *part* of what keeps the timing effortlessly together, not a potential distraction.
If you can keep the roll going while, say, talking or thinking about something else, that's a good sign. Needs to be in muscle memory, like what to do with your feet while walking and also having a conversation. If your feet can do it, so can your fingers, just need enough training!
Now try singing over the roll, and make sure your singing is not "corrupted" by the roll. The roll has to be independent enough so that the normal nuances in your singing are not affected. If you start phrasing your singing according to notes in the roll, that's not good. But this step should work out with practice.
As for putting in nifty licks, try doing that mainly *between* singing phrases:
Roll in my sweet baby's arms.. (nifty lick) Roll in my sweet... etc.
Whatever you find hard to do (you mentioned the harder licks), don't do until and unless you're comfortable. An uncomfortable singer is not who you want singing lead! When singing lead, you're carrying the band, and if the banjo player drops out a bit at that point, well, think of that as a tasteful "doesn't have to be heard all the time" banjo player. You know, Scruggs often could not be heard behind Flatt's singing, and that certainly didn't hurt the music, as it enabled other pickers to be heard more.
With all that said, it should be mentioned that two of my favorite singers over the years have been banjo players, Ralph Stanley and Alan O'Bryant. Both would practically stop playing while singing, leaving the accompaniment to their very competent bands. Some banjo players chop chords the way a mando player would, and there are some who can sing effectively even while rolling and even while doing fills. The late Billy Edwards was famous for that, and he was an excellent singer even while playing full-out banjo. But that is rare.
Hope that helps.