I want to customise my phone to forward SMS messages automatically

You can try to create the following flow with Automate and test if it accomplishes it.

1. Create a new flow by pressing on the + button


2. Press again on the + button, search for SMS. Select "SMS received" block


3. Press the "SMS received" block. Enter the phone number of the sender. Click on "Subscription ID" and select your SIM card (Network provider). On the Message box, enter the name of a variable, for example msg. Click on Save.


4. Press the + button. Select the "SMS send" block. Press it. Enter the phone of the contact you want the SMS to. Set the Sim card (network provider) under "Subscription Id". on the Message box, press on fx to enable it, then enter the variable create during step 3. In this case, enter msg. Click on Save


5. Link the blocks as seen on the first image below, then press back.


6. Give the flow a name and enable "send and view messages". Give the requested permission, then press Start.

When an SMS is received from the sender whose number you entered on step 3, it will be immediately sent to the contact number entered in step 4. The log on the second image shows that an SMS was received then sent (forwarded).

Note: You should be able to add more number during step 3 and 4 by enabling fx. But I have not tested it. Or, you can add more SMS received and SMS send blocks in parallel for each number.

Disclaimer: I am just a user of the app.