Hey, good budget start and nice Scroll Rack :)

Spectral Sailor, Siren Stormtamer, Changeling Outcast, Warkite Marauder, Kitesail Freebooter are all low CMC budget Pirates who have evasion (flying or unblockable). More low CMC Pirates with evasion especially flying makes doing combat damage with three Pirates to a player easier for Brass as well as making more treasures with Malcolm, Keen-Eyed Navigator. Sailor is also a mana sink to draw cards.

• Spectral Sailor > Dire Fleet Interloper
• Siren Stormtamer > Prosperous Pirates
• Changeling Outcast > Brazen Freebooter
• Warkite Maurader > Dire Fleet Neckbreaker
• Kitesail Freebooter > Goblin Trailblazer

Coercive Recruiter and Fathom Fleet Captain have powerful budget repeatable effects with Pirates. Recruiter can help to clear blockers out of way to do combat damage with three Pirates for Brass. Captain can be a repeatable source of Pirate tokens which triggers Recruiter and Merchant Raiders.

• Coercive Recruiter > Headstrong Brute
• Fathom Fleet Captain > Dire Fleet Captain

Malcolm + Lightning-Rig Crew can make a lot of treasures since Malcolm triggers whenever a Pirate does damage to an opponent and Rig does 1 damage to each opponent which creates an amount of treasures equal to the number of opponents you have. You can chain making treasures with Rig by casting Pirates.

Lightning-Rig Crew > Dire Fleet Hoarder

You don't have any instant creature or other permanent removal. Chaos Warp, Reality Shift and Terminate are all budget.

• Chaos Warp > Walk to the Plank
• Reality Shift > Mutiny
• Terminate > Renegade Tactics

Navigator's Compass and Prophetic Prism are not mana rocks; they could be replaced with actual mana rocks to ramp such as Izzet Signet and Dimir Signet. Crumbling Necropolis, Exotic Orchard, Rogue's Passage, Sunken Hollow, Shivan Reef, Grixis Panorama, Ash Barrens are some budget land upgrades to consider.

These 20 card suggestions could be added for about $15. Good luck with your deck.