How to setup NGINX correctly for multiple custom Domains with SSL

I am making a multi-tenant platform. I have my main url as example.com and every new user will get a subdomain with username.example.com this is working. It is running on an Ubuntu droplet on Digital ocean. I want to go one step further and allow them to add custom domains which point to my app by creating a an A name record on their DNS. I got this working as well by setting things up manually and writing additional server block for custom domain. I started with certbot for generating the certificates but then modified a lot of code manually.

Here is what my nginx file look like at /nginx/sites-available/example.com:

server {

    server_name example.com *.example.com;

    # pass to NODEJS app running at :3000
    location / {
            proxy_pass http://localhost:3000;
            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;
    }

    listen [::]:443 ssl ipv6only=on; 
    listen 443 ssl;
    ssl_certificate /etc/letsencrypt/live/example.com-0001/fullchain.pem; 
    ssl_certificate_key /etc/letsencrypt/live/example.com-0001/privkey.pem; 
    include /etc/letsencrypt/options-ssl-nginx.conf; 
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; 
}

server {
    server_name customdomain.com;
    location / {
            proxy_pass http://localhost:3000;
            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;
    }

    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/customdomain.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/customdomain.com/privkey.pem;
    include /etc/letsencrypt/options-ssl-nginx.conf;
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem;

}

server {
    listen 80;
    listen [::]:80;

    server_name ~^(?<subdomain>.+)\.example.com$;
    return 301 https://$subdomain.example.com$request_uri;
}

server {
    listen 80;
    listen [::]:80;

    server_name customdomain.com$;
    return 301 https://customdomain$request_uri;
}

So My questions are :

1. Is there a way I can do this automatically - getting the certificate for custom domain on the fly and allowing it to go to my nodejs app?

2. Should I be creating multiple files under available-domains instead of multiple server blocks in same file ?

   3. Should I just make it under default instead?

3. see that the location block is repeating in every server block, is it possible to do this in a more DRY approach ?

4. I am very new to all this, so if there is a better way to do the multi-tenant setup with SSL and custom domains ?

thank you.