Hacky workaround

RpcApi.js

import { ethers } from 'ethers'

class RpcApi {
  constructor({ endpoint, address, abi }) {
    this._endpoint = endpoint
    this._address = null
    this._abi = null
    this._id = 0
    if (abi && address) this.setContract({ address, abi })
  }

  get _nextId() {
    return ++this._id
  }

  setContract({ address, abi }) {
    this._address = address
    this._abi = abi
    this._iface = new ethers.utils.Interface(abi)
  }

  async balanceOf(address) {
    const balance = await this._call('balanceOf', [address])
    return parseInt(balance)
  }

  async _call(fn, params) {
    return this._fetch('eth_call', [{
      to: this._address,
      data: this._iface.encodeFunctionData(fn, params)
    }])
  }

  async _fetch(method, params = []) {
    const res = await fetch(this._endpoint, {
      method: 'POST',
      body: JSON.stringify({
        jsonrpc: '2.0',
        id: this._nextId,
        method,
        params,
      })
    })

    const { result, error } = await res.json()
    
    if (error) {
      throw new Error(`RPC error: ${error}`)
    }

    return result
  }
}

export default RpcApi

usage.js

const api = new RpcApi({
  endpoint: `https://eth-rinkeby.alchemyapi.io/v2/${ALCHEMY_KEY}`,
  address: contractAddress,
  abi,
})

const balance = await api.balanceOf(address)

Thanks @0x62 yeah I was using the BSC open data seed which requires cors. Which from what I can see you can't apply with cloudflare workers 😢

Just incase anyone else comes here trying to do it with BSC I switched to use https://moralis.io/ for now.