Issue with WordPress Subsite URLs - php

Question

(Sorry for the long post as I'm baffled by many aspects of this issue)
I recently converted my WordPress single-site installation to a multisite network. The former site is now the main site of my network. I'm trying to create multiple subsites for my network but I'm running into unusual issues with the URL.
I have the network setup as a subdomain network. I plan to initially create the subsites as
http://subsite1.maindomain.com
http://subsite2.maindomain.com
But then later map these to
http://www.subsite1.com
http://www.subsite2.com
I'm nowhere close to the domain mapping because even the subdomains are not working properly.
The first issue I encounter upon creating a new subsite...After entering all of the details such as site URL, title, and admin email, I am then presented with a blank white page with no error message. I'm not being redirected to the subsite's dashboard or back to a network page. In order to navigate back, I have to enter my network admin URL directly in my browser in order see the new subsite among my list of sites.
The second issue I encounter is though I set the site URL as "http://subsite1.maindomain.com", when I'm viewing any of my subsites and hover over the Dashboard or Visit link, the URL shows that of my main site, http://www.maindomain.com and not of my subsite. I am able to Edit the subsite and change the basic details but I can't get into the dashboard.
It's important to note that the URLs were working for a brief period of time with subsites that I have since deleted. I would create a new subsite and I would be able to get into the dashboard. But suddenly, with no rhyme or reason, the links started linking to the parent and I lost access to the subsites' dashboards. So then I deleted those subsites using the admin network interface and tried creating new ones but I was never again successful.
I've noticed that with the subsites that I created long ago, WordPress created new database tables for them starting with wp_X (where X was the id of the subsite). Now when I inspect my database, I see no tables that represent any subsites. Perhaps those were tables created by plugins because, as I stated, I was once able to get into my subsites and turn plugins on and off.
Also when I was creating those subsites previously, after adding one I would immediately see the subsite listed under the My Sites dropdown. They no longer appear there.
When I go to edit my subsite, even though I originally entered the URL "http://subsite1.maindomain.com", the URL shows as "://subsite.maindomain.com".This doesn't seem right. And when I try to click "Upgrade Network", I am presented with an issue saying one of the subsite URLs is invalid. I believe this is because of the "://" at the beginning of the URL. I've tried removing this from the URL and resaving it, but it always saves with it.
Everything in my database looks fine.
wp_blogs looks like
blog_id
site_id
domain
path
1
1
www.maindomain.com
/
15
1
subsite1.maindomain.com
/
no data in blogs_meta
wp_site has only one record
id
domain
path
1
www.maindomain.com
/
wp_sitemeta contains a few thousand records but the site_id column for all of them is 1. The value for siteurl is https://www.maindomain.com/
wp_config.php
define( 'WP_ALLOW_MULTISITE', true );
define( 'MULTISITE', true );
define( 'SUBDOMAIN_INSTALL', true);
define( 'DOMAIN_CURRENT_SITE', 'www.maindomain.com' );
define( 'PATH_CURRENT_SITE', '/' );
define( 'SITE_ID_CURRENT_SITE', 1 );
define( 'BLOG_ID_CURRENT_SITE', 1 );
.htaccess
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]
I thought maybe the issue could be with SSL or DNS, but the URLs should still link and display the correct link when I hover over the Visit or Dashboard links.
I've deactivated most of my plugins network-wide and really don't want to deactivate any more plugins or my main theme as it will cause my main site to be down until they are activated on the site level. In my experience, reactivating my theme once I deactivated doesn't always honor my previous settings. I don't think the issue is a plugin or theme conflict.