Your strongest cards support:

self-mill
recursive value
ETB creatures
token generation
attrition
grindy inevitability

The Spider package works best as the defensive shell around that engine.

The BEST additions from your pool

These are the cards I’d prioritize first.

Auto-includes

These genuinely improve the deck a lot.

Eldritch Evolution

This is excellent with:

Solemn Simulacrum
Springbloom Druid
Elvish Visionary
Dusk Legion Zealot
Burnished Hart

Turns small value creatures into:

Ishkanah
Rune-Scarred Demon
Massacre Wurm
Prime Speaker Zegana

Very strong here.

Strategic Planning

Exactly what Muldrotha wants:

card selection
graveyard filling
cheap setup
Crow of Dark Tidings

Looks mediocre, plays surprisingly well in Muldrotha.
Mill on ETB and death is great recursion fuel.

Noose Constrictor

Underrated graveyard enabler.
Lets you pitch:

Spider Spawning
Rise of the Dark Realms
reanimation targets
excess lands

at instant speed.

Whisper, Blood Liturgist

This is VERY good in your deck.
You already make tokens and disposable creatures.

Recurring:

Massacre Wurm
Rune-Scarred Demon
Ishkanah
is nasty.
Vampiric Rites

Perfect grind engine.
Works beautifully with:

Spider tokens
Solemn Simulacrum
recursive creatures
Muldrotha replay loops
Dread Summons

This may honestly become one of your strongest cards.

It:

mills everyone
fuels Consuming Aberration
loads your graveyard
creates zombies
enables Rise of the Dark Realms

Excellent fit.

Spider package worth adding

These I DO like:

Swift Spinner

Efficient spider with reach and hexproof on your turn.

Treetop Snarespinner

Actually solid.
Makes spider tokens trade up well.

Canopy Spider

Low power level, but okay if you really want more spiders.

Giant Spider

Classic, but probably below your deck’s power level now.

REALLY strong synergy cards you own

These aren’t spiders, but they improve the deck massively.

Rite of Replication

Absurd with:

Massacre Wurm
Rune-Scarred Demon
Ravenous Chupacabra
Prime Speaker Zegana

This card wins games.

Branching Evolution

Only if you lean harder into counters.
Right now I’d skip it.

Wishclaw Talisman

Very playable in slower casual metas.
You can recur it with Muldrotha after sacrificing/bouncing it.

Fierce Empath

Excellent tutor here.
Finds:

Massacre Wurm
Rune-Scarred Demon
Aggressive Mammoth
Pelakka Wurm
Pelakka Wurm

This card is incredible in recursion decks.
Life gain + card draw + big body repeatedly.

Cards I would CUT first

These are your weakest synergy pieces currently:

Easy cuts
Voracious Greatshark
Primeval Bounty
Negate
Soaring Seacliff
Underwater Tunnel // Slimy Aquarium
Valgavoth’s Lair
Hero’s Downfall (keep one if desired)
Gilded Lotus
Mazemind Tome
Rogue’s Passage
Temple of Deceit (optional)
Temple of Mystery (optional)
One important thing:

You have enough support to make:

Spider Spawning

a REAL win condition.

That card gets terrifying when combined with:

self-mill
Dread Summons
Crow of Dark Tidings
Noose Constrictor
Perpetual Timepiece
Eccentric Farmer

You could realistically create:

15–30 spiders
then either:
alpha strike
sac them for value
recur everything afterward

That’s a legitimate game plan.

My recommendation overall
Lean into:
graveyard setup
recursive value
spider tokens
ETB creatures
sacrifice/card draw engines
Avoid:
random standalone bombs
reactive counterspells
expensive non-synergy artifacts
too many Gates

You’re honestly getting pretty close to a very annoying grind deck that becomes hard to kill after turn 7–8.

Underwater Tunnel // Slimy Aquarium (what is this doing in here anyway)

Honestly, in this deck? Not much.

Underwater Tunnel // Slimy Aquarium is probably in that awkward category of:

“technically synergistic, practically underperforming.”

What it tries to do here:

enters as a blue source early
later becomes a mana sink
self-mills a little
eventually makes a token

But for a Muldrotha graveyard deck, it’s inefficient at basically every role.

The problems
Comes in tapped
Very slow payoff
Milling is tiny relative to what your deck wants
Token production is delayed and mana-intensive
Doesn’t synergize strongly with recursion loops
Takes a land slot while functioning like a weak spell

Compared to your better enablers:

Eccentric Farmer
Nyx Weaver
Spider Spawning
Dread Summons
Perpetual Timepiece

…it’s just low impact.

This is exactly the kind of card I’d cut for:

a self-mill creature
sac outlet
recursion spell
ramp creature
stronger spider payoff

So your instinct is correct. It doesn’t really advance your main plan effectively.