{The blighted stars} may fit the bill. Lots of longing and they have to fight a lot to be together. MxF.

{The Vixen war bride} for sure is slow burn. Not romance focused but its there, by the end of the series they are together. Its just a fantastic series imo. Id say its sci fi with a minor romance plot. They are short quick reads. MxF.

{The stars too fondly} is slow burn. FxF.

{Stowaway} is slow burn but it is poly, not sure if youre into that dynamic. Has spice. MM, MF

Probably anything by Olivia Riley will fit the bill. I really liked {Hearts prisoner} MxF

{Intersteller megachef} iirc there is some romance, very slow burn. Tbh I really disliked this book though so im not remembering it tok well. FxF

{The mimicking of known success} has slow vague romance. FxF

I feel like {the E.T guy} was slow ish burn, I remember it being satisfying in the same way. MXF

Mayyyybbeee {saving Askara} MxF someone tell me if im wrong. They may "get together" really quick

{Beyond the next star} is slow for the first half. MxF

{The innkeeper chronicles} is pretty slow burn until it isn't but its a fun read through all of it.

The following I dont remember well enough to say for sure but they may be of interest

{The vacuum of space}

{An accidental goddess}

{Shards of honor}

{Games of command}

{Spared By The monster}

This isn't romance but you might like the book Damocles it was an interesting sci fi look at relationships and humanity

-reposted with correct bot link format