"it didnt start with the instant spark that some people seem to need"
I read recently that "the spark" is not automatically evidence of positive chemistry, it can actually be anxiety caused by threatening male behavior (e.g. dominance, unpredictability, jealousy, or boundary-crossing), but women have been trained by patriarchy to mislabel it as sexual attraction.
The body’s first response to excitement and fear is similar - a racing heart, faster breathing, tense muscles, sweating, and high alertness. The brain then distinguishes between the two - excitement and fear - by looking for context.
But in patriarchy, the context is blurred, because girls and young women are told that anxiety-inducing male behaviour is desirable. They're trained to not listen to their fear.
So a woman who feels anxious because of a man's pushy behaviour (for example) may actually think she is turned on by him. This phenomenon is called misattribution of arousal. It was neatly demonstrated by the Capilano Suspension Bridge study, which showed that men who crossed a high swaying suspension bridge (which caused intense anxiety) were significantly more likely to feel romantically attracted to the interviewer afterwards than men on a safe stable bridge.
So in fact, feeling "the spark" could actually be a red flag.
Feeling that you can be yourself and in fact not feeling your nervous system is on fire are probably better signs of true compatibility.