Alex

Investigation jdramas.
Set in departments that are looked down upon/essentially forgotten by the rest of the police.
— Aren't taken seriously, but are productive nonetheless.
Quirky chaotic female leads who annoy cold/rude male leads.
Both feature Ryu Raita in a supporting role.

Keizoku is apparently a parent story to SPEC though I see no direct correlation in story. SPEC leaned into supernatural abilities while Keizoku aimed to disprove and rationalize.

Both legal dramas.
Both feature a mentor/mentee dynamic between female lead and an older female boss.
Both feature strong and interesting female characters, all different in personality.
Subtle/slight romance.

Age gap between leads.
Celebrity male lead.
Leads converse online while not knowing they have frequently met in real life.

Age gap between leads.
Male lead is an actor with a bratty personality.
There's a single mom character and the paternity of her child is questioned.

Korean thrillers.
Both about kidnappings.
Father of the child is/becomes the main focus.
[ SPOILER ] The kids end up dying in both.

Prison setting.
Female led/driven plots.
Female leads are innocent of the crimes that sent them to prison.

Both dark comedies.
Both focus on families facing work/business struggles.
Death and murder soon overtake the stories.

Both darker themed detective jdramas.
Male leads hold a secret while investigating.
Both have a supporting female character who takes an interest in male lead.

Both plots revolve around supernatural abilities.
Both star Kamiki Ryunosuke & Arimura Kasumi (minor roles in SPEC, leading roles in FE).

A hitman falls in love with a woman.
Helps her with her passion projects- art/music.
Wants to be better and leave line of work for her.
[ SPOILER ] Female lead dies in the end.

Noble/rich male lead thinks the woman he loves has died.
Years later he comes across a lowborn/poor girl who looks identical to her.
Female lead has lost her memories and does not know she truly is male lead's long lost love.

Cold and closed off guys who prefer to work alone are assigned a new cheery and excited partner.
They prefer to stay in the "basement" while their new employee does most of the work outside.