well stated, my brother.
you understand what we were promised...and what we lost between the initial trailers of TFA to the end of RoTS. We were |---| close.
Right? As much I really do love The Last Jedi (in particular what Johnson did with Rey, Luke, and Ben), I was so gutted when Finn was shuffled off to the side, and there was no follow through on, what I felt, were obvious teases in TFA that he was Force Sensitive. I mean, there wasn't even any follow through on his conflict with Phasma. Don't get me started on that, but one of the ways I would have handled Finn and Rose's arc in TLJ would have been:
The setup is the same: They go to Canto Bight to find the codebreaker, but Phasma, who is so incredibly diligent and vengeful (and maybe I'd tease she has a little Force sensitivity herself), happens to notice the smallest warp signature leave the capital Resistance ship. She feels it in her bones that it's FN-2187, and because she hates that fucker, she leaves Snoke's ship without informing anyone of her plan (she wants to dispose of him herself), and tracks them to Canto Bight (shades of Boba Fett in Empire). Finn and Rose arrive on Canto, and instead of being arrested and thrown in jail, they are doggedly pursued by Phasma, and her Squad of loyal soldiers. It then becomes this cat and mouse scenario, where Finn and Rose are trying to keep one step ahead of Phasma, while also trying to accomplish their mission. I'd focus on stressing the tension between the importance of their mission, and how close Phasma is to undermining it.
During this, we get to learn more about Finn, his history with the FO, delve a little deeper into his vague memories of a life before he was conscripted, and maybe get a little more teases that his Force potential is awakening. Bonus, we learn more about his beef with Phasma, and why he was so terrified of her, and, if he's honest, why he still is. We can also use this time to bond him to Rose. Her upbringing was very different, and we learn a bit more about her relationshp with her sister, as well as her disdain for the big wigs of Canto Bight (one of the aspects of the Canto Bight sequence I really liked). Both characters get some much needed depth, interspersed with Rey's struggle to snap Luke out of his funk, and Poe's lesson in humility and leadership. They actually meet the codebreaker, and just as he's swayed to join their cause, Phasma catches up to them and kills the codebreaker. Their mission was a failure, but it wasn't a wasted sidestory, as we got to learn more about Finn and Rose, and see them progress their individual arcs a little bit.
During this confrontation with Phasma, I'd have Finn do what he did in the deleted scene. He reaches out to the her Stormtrooper Squad, just as things looks bleak, and he actually manages to sway a few of them! As implied in Rise, some of the Stormtroopers are Force sensitive, and they recognize some kind of kinship in Finn. Not all of the troopers lower their weapons, so Phasma and her loyal squad turn on the traitors, and a firefight breaks out, allowing Finn and Rose to flee. It is here that I'd have them encounter DJ, setting up that plot thread of the shady scoundrel who encourages Finn to just walk away. The escape from Canto Bight plays out in the same way (well, the theme park ride chase is excised in favor of the Phasma/Finn/Rose/Stormtroopers segment), and DJ flies them out on the stolen ship.
The rest of their arc plays out roughly the same. They get captured and betrayed by DJ on Snoke's snip trying to disable the Hyperspace tracker, and Finn is slowly beginning to understand that he may have another way to fight the First Order. Even though they failed, he feels like he may be able to convince more Stormtroopers to turn against the First Order. He's going to trust in the Force, and Rose is going to join him in that fight as well, for her own sense of wanting to matter, and do right by her sister's legacy.
When we pick up that thread in Rise, it's been a year, and while he's not a Jedi, he is more adept at Force usage than we've seen. He doesn't have his own lightsaber...yet, but he's displaying some pretty impressive knowledge of the Force. Also during that year, he and Rose have managed to dig up intel on potential Stormtrooper defectors, and have swayed a few squads to the cause, but not nearly enough. They need more. He and Rose go with Rey and Poe to find the Sith wayfinder, but during Chewbacca's rescue attempt, he converts a few more Stormtroopers, and they crash land on the planet with the Death Star remains. He meets Jannah as written, believing truly that the will of the Force put him on this path. His newly converted Stormtroopers, and her group of defected Stormtroopers align and their role in the final battle plays out similarly. Then, after Palpatine is finally defeated, he decides he's going to continue traveling to various star systems, finding any remaining remnants of the First Order, and trying to convert them. He wants to continue his study of the Force, and that's where Maz Kanata comes in. She's no Jedi, as she says, but she does know the Force, and will take it upon herself to teach him what she knows. Maybe have a tease of him looking at the old Jedi texts Rey has on how to construct his own lightsaber when he's ready for it.
I mean, that's all I wanted from Finn in this trilogy. He's a likable character, was always meant to be Force Sensitive, and his journey didn't necessarily have to be the same as Rey's, but he still gets there in the end. Mace Windu was Sam Jackson with a lightsaber. That's not necessarily a bad thing, but I definitely like Finn, and wanted better for him by this trilogy.
I'd love it if Lucasfilm could convince Boyega to return to the character for a short Disney + miniseries. Like, 6 episodes, where we get to see him grow his Force abilities and progress his journey to becoming a Jedi. Have a cameo by Daisy Ridley and Oscar Isaac too, if you want. They have excellent chemistry, so an episode where a more experienced Rey and Finn, along with Poe, have a team up adventure for whatever plotline Finn is on would be awesome.