To keep the original source port unchanged if possible is the default behavior of the Mikrotik firewall. "If possible" means "if no other connection to the same remote IP address and port already uses the same port on the new local IP address".
If you set the to-ports parameter of an action=src-nat or action=masquerade rule to a single port number, and that port is already occupied by another connection from the same new local IP address to the same remote IP address and port when the initial packet of a new connection arrives, this initial packet is dropped - even if the to-addresses contain a list of addresses to choose from.
Instead of writing novels, post /export hide-sensitive. Use find&replace in your favourite text editor to systematically replace all occurrences of each public IP address potentially identifying you by a distinctive pattern such as my.public.ip.1.