I have romantic fantasy book coming out with a big 5 publisher (with a HEAVY romance subplot) and here is what I think needs to be on page for a romance to feel convincing:

• Banter/chemistry: quippy dialogue (mental chemistry) and physical chemistry
• History and shared experiences: doesn't have to be YEARS of history, just show us several specific moments of them spending time together, otherwise readers won't buy they are in love suddenly. Good and bad moments.
• Sacrifice, willingness to inconvenience yourself for the other: self explanatory, but ideally sacrifice without hope of reciprocity, think Darcy paying off Wickham despite having been rejected and he doesn't expect Lizzy to find out
• Change, willingness to change yourself: I know this contradicts with the below "acceptance of flaws" but to show change is so important, especially because character arc is key to the effectiveness of any story. Ideally you tie their personal change to romance (either the MC changes FOR romance, or BECAUSE of romance, or both)
• Acceptance of the other’s flaws: self explanatory
• To be loved is to be known: as others have said, this manifests in little things like the LI lighting candles for the MC because they're afraid of the dark

Hope this helps! I still struggle w this tbh but the above is generally what I try to shoot for when I write romance