This might be a bit on the short side for what you're looking for, but I think it fulfils all your other requirements:
The Old Bad Songs (20849 words) by fengirl88 Chapters: 11/11 Fandom: Sherlock (TV), Maurice (1987) Rating: Mature Warnings: No Archive Warnings Apply Relationships: Sherlock Holmes/John Watson, Sherlock Holmes/Lestrade (Inspector), Lestrade (Inspector)/Maurice Hall Additional Tags: Mystery, Romance, Angst, Crossover, E.M. Forster - Freeform, Crossover Pairing, Queer History Series: Part 1 of The Old Bad Songs and other stories Summary:
Lestrade becomes enmeshed in a blackmail case he is working and has to turn to Sherlock for help.
And there's a series of short fics exploring the endgame relationship, too.
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timeasmymeasure
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Date: 2017-10-26 07:54 pm (UTC)The Old Bad Songs (20849 words) by fengirl88
Chapters: 11/11
Fandom: Sherlock (TV), Maurice (1987)
Rating: Mature
Warnings: No Archive Warnings Apply
Relationships: Sherlock Holmes/John Watson, Sherlock Holmes/Lestrade (Inspector), Lestrade (Inspector)/Maurice Hall
Additional Tags: Mystery, Romance, Angst, Crossover, E.M. Forster - Freeform, Crossover Pairing, Queer History
Series: Part 1 of The Old Bad Songs and other stories
Summary:
Lestrade becomes enmeshed in a blackmail case he is working and has to turn to Sherlock for help.
And there's a series of short fics exploring the endgame relationship, too.