Get Completionist Cape and Master Quest Cape first both are tracked as prerequisites on this cape's tab — Trimmed Completionist is built directly on top of them, not a separate grind from scratch.
The requirement list grows over time as new content releases, Jagex periodically adds new achievements to the trim list — this is the most "living" of the four capes.
84 trim-exclusive achievements these are on top of (not instead of) everything already required for Completionist Cape and Master Quest Cape, which are tracked on their own tabs.
Buying isn't a separate requirement once Completionist Cape, Master Quest Cape and everything above are met, purchase from the same Museum guard for 5,000,000 coins.
Achievements (84)
Activities
Build Them All! — Build all of the Treasure Trail hidey-holes. You can find a complete list of hidey-holes at the noticeboard by Zaida. (~45 min)
Champions. What Champions? — Defeat all champions in the Champions' Challenge. (12-20 hrs)
Eastern Explorer — Explore the Eastern Lands as far as the Shield from your port. Distance travelled: (X/4,100,000) (4-7 hrs)
Get Tooled Up — Fill up your tool belt. (~1 hr)
Globetrotter — Purchase the full Globetrotter outfit. (7-12 hrs)
Jack of the Circus — Obtain all circus clothing from Balthazar Beauregard's Big Top Bonanza. (8 weeks)
Master Angler — Purchase the champion's tackle box from the Fish Flingers fisherman. (2-3 hrs)
Master Otto — Complete all barbarian training with Otto, at his grotto west of Baxtorian Falls. (1-2 hrs)
Nomad's Mirage — Defeat the memory of Nomad and loot one of his bounty chests. (~1 hr)
Out of this World — Unlock everything in the Shooting Star Reward Shop. (X/3) (~27 hrs)
Penguin Adoption Agency — Adopt Sheldon the penguin by tracking penguin spies around the world. (6 weeks)
Raptor's Revenge — Complete the Ungael combat activity on hard mode. (~15 min)
Scrolling in the Deep — Find every scroll piece in Player Owned Ports. (~10 min)
Combat
A Song of Ice and Ire II — Defeat the Arch-Glacor at 100% enrage. (X/100) (~15 min)
Conquered Everyone's Heart — Reach maximum reputation with all Heart of Gielinor factions. (133 days)
Death Effect — Unlock all permanent effects from Death's Store. (X/8) (~24 hrs)
Finish Him! — Personally defeat Vorago with the Maul of Omens. (1-2 hrs)
Jail break — Free each of the baby dinosaurs from the jail cells within the Rex Matriarch lair. (2-3 hrs)
My Goebie Homies — Obtain maximum reputation with the goebies on Mazcab. (~1 hr)
Reaper Crew — Defeat listed bosses at least once. (X/55) (~10 min)
Shadows Below — Survive the shadow beneath the barrows. (7-12 hrs)
Telos If You're Angry II — Defeat Telos at 100% enrage. (X/100) (~15 min)
War. What Is It Good For? — Buy all unlocks and upgrades from War's store. (X/2) (~10 min)
Exploration
All Your Energies — Unlock the energy-gathering scrimshaw scroll from Waiko ports reward shop. (~30 min)
Bone to Pick With You — Unlock the bonecrusher upgrade from Waiko reward shop. (~30 min)
Canoe Fix It? Yes You Can! — Repair all the canoe stations around Havenhythe. (X/4) (~10 min)
Don't Crush My Memories — Unlock the memory-crushing scrimshaw scroll from the Waiko ports reward shop. (~30 min)
Elder Wisdom — Earn the Wisdom of Anima fletching perk for all the Elder God arrow types. (~1 hr)
God of the Grind — Fully repair the statue of Het. (1-2 hrs)
Jurassic Perk — Discover all the totem pieces on Anachronia. (X/7) (2-3 hrs)
Master Builder — Fully upgrade the base camp on Anachronia. (65 days)
Mitts and Waders - Death Lotus — Unlock the death lotus gloves and boots scrolls from the Waiko reward shop. (~45 min)
Mitts and Waders - Seasinger — Unlock the seasinger gloves and boots scrolls from the Waiko reward shop. (~45 min)
Mitts and Waders - Tetsu — Unlock the tetsu gloves and boots scrolls from the Waiko reward shop. (~45 min)
Remotely Possible — Build the Remote Totem Access Point in the base camp on Anachronia. (~15 min)
Reputation - Class Crown — Earn tier 10 reputation with the imperial faction in Menaphos. (4-7 hrs)
Reputation - Lock, Stock and Barrel — Earn tier 10 reputation with the merchant faction in Menaphos. (4-7 hrs)
Reputation - Seagullible — Earn tier 10 reputation with the ports faction in Menaphos. (4-7 hrs)
Reputation - To the Bone — Earn tier 10 reputation with the worker faction in Menaphos. (4-7 hrs)
Salty — Fully explore what the Arc has to offer, and earn the right to claim the 'Salty' title. (2-3 hrs)
Sandy — Complete various achievements within Menaphos... and then some more to earn the right to claim the 'Sandy' title. (~10 min)
Sort of Crystally — Obtain the 'Dark Lord' title by unlocking a selection of titles from Prifddinas. (1-2 hrs)
The Golden Crocodile Hunter — Unlock all Hunter upgrades from Dundee's Shop. (1-2 hrs)
Um Pop — Meet all of the listed inhabitants of Um. (~1 hr)
Who Will Be Her Lover? — Complete the replica statue of Rhiannon in the Max Guild garden. (12-20 hrs)