Intermittent cookies error after contribution

We experience a cookies error intermittently after submitting a donation. Currently on Civi 5.35.2 and WP 5.7.2. This is multisite so may be multisite related.

A contributor does not get to the thank you page but gets the error above. The donation does succeed. This issue has persisted for a long-time (2+ years) and is a probably the worst civi issue I've encountered seeing it has to do with contribution pages and has no pattern that we can find. Tried all the go to solutions posted on SE with no luck. I would estimate that it occurs ~10-20% of the time.

This is related: #517 (closed) and says using PHP 7.2 resolved, but were on 7.2 and now on 7.3

Some steps I've done:

• Set Clean-up Temporary Data and Files scheduled job run frequency from Hourly to Daily. It is on every site.

• Changed cookie domain constant in wp-config.php

// define('COOKIE_DOMAIN', false);
to
define('COOKIE_DOMAIN', $_SERVER\['HTTP_HOST'\] );

• Enabled https://lab.civicrm.org/extensions/qfsessionwarning and set timeout at 120 minutes. $civicrm_setting['core']['secure_cache_timeout_minutes'] = 120;

We have considered changing how garbage collection works as sessions will die if garbage collection is run (server does that every 15 minutes).

This post seems to have found a solution using the Fatal Error Handler. Is this recommended and how do you implement? Seems like a workaround symptom fix 'solution'.

Also getting this for additional event participants all the time (usually) or not at all depending on when I test: #2708 (closed)

Help?