thank you to all.
Export / Import of the config should work without a problem.
Don`t worry !
Thank you very much, if I make a backup from Files → Backup, is the same?
You cannot load a “backup” on another device than where it was made.
It is possible to import an “export”, but it may require some minor tweaks.
And of course, the config of an RB750Gr3 is not complete for a hAP ac2, because it misses the wireless part.
You will have to configure that afterwards.
When possible, please start from defaults. Especially after many years, there will be improvements in the defaults that you will not
have when you start from an old config.
Export the RB750 config and keep it as a guide when configuring the new router. For large and complicated sections, you can cut/paste it.
But especially for firewall it is better to start from defaults.