Personally, I have always enjoyed playing gnomes, but they never felt quite right for evil character concepts. Even gnome tieflings only partially fill that niche. Svirfneblin, while starting in Andunor, lean heavily into neutrality and lack the malicious undertones present in most monstrous races. They feel completely disconnected from the more ruthless denizens of the Underdark.
Seeing how Spriggans NPC gained their lore ability quite recently, I feel like it's worth a shot - so here's my suggestion:
Evil Alignment
+2 Dexterity, +2 Constitution, –2 Charisma
Small-sized
Darkvision
Keen Senses
Innate ability: Distorted Growth (1x day)
Spriggans grow to Medium/Large size for 3 rounds
Potential penalties to stealth when grown, unsure about bonuses