Have you tried using Lets Encrypt with certbot to install the SSL certificate in the nginx reverse proxy? I saw on the github link above that you can use nginx as reverse proxy.

Trillium can be installed in Digital Ocean server with Docker setup or can be done using the manual installation. Trillium is running on port 8080 and we need to proxy pass with Nginx to run it under your domain name.
Rgearding SSL issues, the domain needs to be setup with normal nginx configuration and we can use paid SSL or Free SSL (Let’s Encrypt). Then proxy pass port 8080 to the domain with the below conf as mentioned in Trillium setup and it will work properly.

location /trilium/ {
    proxy_pass http://127.0.0.1:8080/;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection ‘upgrade’;
    proxy_set_header Host $host;
    proxy_cache_bypass $http_upgrade;
}