we can use a simple formula that gives minimum swap fee based on the amount of staked tokens:

Swap Fee = Fmin + (Fmax−Fmin) × (C / (C+S))
where:
Fmax is the maximum fee percentage (e.g., 1% when no tokens were staked)
Fmin​ is the minimum fee percentage (e.g., 0.1% as the lowest possible fee).
C is the constant that determines how fast the fee decreases with increased staking (adjustable).
S is the amount of tokens the customer has staked.

a few examples:
user staked no tokens (S = 0): Swap Fee = 0.1% + 0.9% × (800 / (800 + 0)) = 1%
user staked 700 tokens (S = 700): Swap Fee = 0.1% + 0.9% × (800 / 1500) = 0.58%
user staked 800 tokens (S = 800): Swap Fee =0.1% + 0.9% × (800 / 1600) = 0.55%
user staked 1200 tokens (S = 1200): Swap Fee = 0.1% + 0.9% × (800 / 2000) = 0.46%

we can calculate this on each swap. The actual change can happen in the pool contract
where:

let compute_swap: ComputeSwap = compute_swap(
    &env,
    pool_balance_sell,
    pool_balance_buy,
    offer_amount,
    config.protocol_fee_rate(), // we change this input parameter to be calculated before this function call
    referral_fee_bps,
);

'C' should be configurable by admin at any time. Ideally, we should somehow automatically adjust the value based on the staked amount in the pool(s)

another approach to this problem would be if we actually use the staking contract and keep the variable fee in its storage. We can update the calculation whenever user does stake/unstake and then in pool just check for the %