How to send a multiline string parameter as a command via AWS SSM command?

I am trying to create an .env file on my remote AWS instance using a Github Workflow file. I am using the following command to set the contents of the .env file:

  - name: Setting Github env vars to variable
    run: |
      echo "DEV_ENV_VAR<<EOF" >> $GITHUB_ENV
      echo "$VARS_JSON" | jq -r 'keys[] as $k | "\($k)=\(.[$k])"' >> $GITHUB_ENV
      echo "EOF" >> $GITHUB_ENV

This creates a multiline string of data that needs to be put into an .env file. I am then trying to do the following command:

unbuffer aws ssm start-session --parameters command="cd /web/app; git pull; yarn install; echo ${{ env.DEV_ENV_VAR }} >> .env; yarn build;" --target '${{ env.MY_TARGET }}' --document-name "AWS-StartInteractiveCommand";

But it doesn't seem to write anything into the .env file, and gives a syntax error if I am using a -e parameter for echo command. How do I correctly achieve sending the multiline string in the parameters? Is there perhaps another way of achieving my goal?

My whole workflow file looks like this right now:

name: Update Frontend

on:
  push:
    branches:
      - develop

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true

permissions:
  id-token: write
  contents: read

jobs:
  deploy:
    runs-on: ${{ matrix.operating-system }}
    strategy:
      matrix:
        operating-system: [ubuntu-20.04]
    environment: dev
    env:
      DEPLOY_USERNAME: ubuntu
      DEPLOY_SERVER: (my AWS instance ID here)
      DEPLOY_PATH: /deploy/path
      VARS_JSON: ${{ toJSON(vars) }}
    steps:
      - name: Configure AWS Credentials
        uses: aws-actions/configure-aws-credentials@v2
        with:
          aws-region: eu-west-1
          role-to-assume: ${{ secrets.AWS_ROLE_TO_ASSUME }}
      - name: Setting Github env vars to variable
        run: |
          echo "DEV_ENV_VAR<<EOF" >> $GITHUB_ENV
          echo "$VARS_JSON" | jq -r 'keys[] as $k | "\($k)=\(.[$k])"' >> $GITHUB_ENV
          echo "EOF" >> $GITHUB_ENV
      - name: Running commands on server
        run: |
          sudo apt-get update;
          sudo apt-get install expect -qq > /dev/null;
          echo "Starting execution of commands";
          unbuffer aws ssm start-session --parameters command="cd ${{ env.DEPLOY_PATH }}; git pull; yarn install --frozen-lockfile; echo ${{ env.DEV_ENV_VAR }} >> .env; yarn build; pm2 restart all;" --target '${{ env.DEPLOY_SERVER }}' --document-name "AWS-StartInteractiveCommand";