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Your problem is server configuration, not how Akeeba Backup, Joomla or PHP works. 

The utf8mb4_unicode_ci encoding is a new encoding added in MySQL 8. It uses a much newer version of Unicode (9.0.0) than what MySQL 5.5, 5.6 and 6.7 used (4.2.0).

Since this is your default database encoding, all your tables now claim to be encoded in the utf8mb4_unicode_ci encoding. Trying to run that DDL SQL on MySQL 5.x predictably fails.

Akeeba Backup cannot currently convert the claimed utf8mb4_unicode_ci table encoding to utf8mb4_unicode_ci. I am not sure it should even try to. Downgrading the encoding could have some unpredictable sorting effects for content authored in languages that use diacritics and accents.

Furthermore, Joomla only supports the utf8mb4_unicode_ci encoding. Using a different encoding in your database could have unpredictable effects in its sorting order -- which is why you see Issue Tracker entries about ordering that nobody else can reproduce...

The correct approach is for you to change the encoding of your databases and your tables to utf8mb4_unicode_ci, the encoding supported by Joomla. I am still unsure whether I should support an automatic downgrade. I will need to think about it and take into consideration what we currently do for UTF8MB4 data when restoring it to plain old UTF8 servers (MySQL versions older than 5.5).

Seeing that we are already downgrading utf8mb4_unicode_ci to utf8mb4_unicode_ci on MySQL 5.5 I will add similar code to downgrade from MySQL 8's utf8mb4_unicode_ci encoding to utf8mb4_unicode_ci on MySQL 5.7/5.6 or utf8mb4_unicode_ci (on MySQL 5.5).