Illusions and shadow magic are heavily dependent on spell level, for saves and effects. A bard isn't the best at them. An umbral sorcerer isn't the only option though; there are things like an evangelist cleric going into the veiled illusionist prestige class, or a witch with the shadow patron and hexes chosen for buffing, or a mesmerist with the material manipulator or umbral mesmerist archetypes, or other bloodlines of sorcerer can buff better while simply picking shadow and illusion spells as spells known.
1. Brazen Deceiver is a: Good Archetype. But it doesn't get many shadow-flavored features for quite some time.
2. Shadow Caller Spiritualist comes to mind as a good option too – buffing and illusion are built into the kit, but the illusion side is a bit weak.
3. Shadowbound Curse, Shadow Mystery Oracle stands out as a great mix of everything you wanted – the illusion spells, the buffing kit. You can toss in Pei Zin Practitioner for condition removal too!
My GM played a shadow-themed bard before I joined the group. I'll ask him tonight about it. I don't think they got past level 5-ish, but he might have some good ideas.
Alternatively you could try an Occultist with a focus on Illusion. The Resonant Power isn't specifically shadow-themed, but could easily be flavoured that way, and at level 9 you can get a pretty-much-all-day summoned shadow (which is actually really good).
Take a look at puppetmaster magus as well. All the bard list, magus list and a chance to grab wizard spells via arcana.