ReadOnly being a reserved word in Php 8.1 causes the page to throw a compilation error (there is apparently an exception in 8.1 which allows the term to still be used as Function Name, but not as a Constant).
A ) GitHub listed a modification (changing the constant to "ReadOnlyProperty") which is included in the most recent fileset. I downloaded the current fileset and swapped it for that in the existing plugin code and that error went away.
B ) There were still numerous deprecated warnings regarding Serialize and Unserialize. These were easily tracked down, modifying those classes by changing method names from serialize to __serialize and unserialize to __unserialize, then removing the "implements Serialize" and "implements Unserialize" from the class declarations.
C ) XmlDriver and YamlDriver both had methods which did not match the formats of their class "Extending" method declarations, so removing the "string" clarification for last input variable made them match.
D ) There was an errors in /plugins/hikashoppayment/authorizejs/lib/net/authorize/util/ANetSensitiveFields.php where null values were being supplied to foreach statements, which I patched by changing
foreach($sensitiveTags as $sensitiveTag)
to
foreach($sensitiveTags ?? [] as $sensitiveTag)
Which I believe is a backward incompatible fix using PHP 8.1 . I don't think most of the changes I made are backward compatible, but it seems to be stable using Php 8.1 now.