Solution for SSL VPN Configuration Renewal Without Requiring User Re-Download

You could replace the VPN Portal Certificate with the public cert - do not replace the SSLVPN Signing cert - There is no benefit in doing it. 

When I change the portal certificate, there is no need to download the configuration. But if I don't replace the VPN app's certificate with a new one, the VPN doesn't work.

Yes they will have to reload the certificate, it will break all existing VPNs, but this is a one time change, you would do and not be an yearly change. 

By the way: Sophos Connect supports a provisioning file: https://docs.sophos.com/nsg/sophos-firewall/21.0/help/en-us/webhelp/onlinehelp/AdministratorHelp/RemoteAccessVPN/IPsecSSL/SophosConnect/RAVPNSConProvisioningFile/index.html 

Having approximately 1000 users install certificates creates a lot of problems. :) there must be another solution

Again: Using the yearly certificate creates a profile, which is usable only a year - And there is no real benefit of doing it with a public signed certificate. 

You need to switch it back to the appliance certificate, which is valid for a long time (15 years or so) and redeploy it. You could redeploy it by using the Provisionen file above. 

If needed, you can also contact your Sophos Partner for more insights. 

I understand, but it is not possible for me to distribute the device's CA private certificate (i.e. Appliance Certificate) to users' computers. Because users are customers working outside my network. Therefore, it is not possible for me to publish the certificate.

Why would you publish this? 

Let me recap what is important: 

1 User logins to VPN Portal: Here you use your public cert, as it is only the VPN Portal via HTTPS.
2 User downloads the OVPN File: Here you get the public key of the self-signed cert + a User cert + private key for user cert. (That is included in the OVPN). 
3 User imports this OVPN into a VPN Client. 

If you use Sophos Connect VPN Client, you could replace the step 1 & 2 with the provisioning file. 

I repeat my question. When I choose this, do I need to manually add this CERTIFICATE to user computers?

No. 

BUT: You need to redeploy all OVPN Files. 

very good ! thank you very much

Please make a backup before this change. 
Because you will destroy all existing VPN Clients after saving the new Certificate. All VPN Clients will need a new config. 

Yes, I will ask all our users to download from the portal again.

Yes, I will ask all our users to download from the portal again.