On Sailfish OS you have Android AppSupport and you can use MicroG. Many banking apps do work that way, but not all (here is a list of tested banking apps for Sailfish OS compatibility: Banking apps on Sailfish OS). EDIT: You can try what Ikraav linked to below though.
If you use Google services, you hand over a lot of your data, no matter what OS or degoogled Android system you are using, simply because that is required for those applications to function. The only way you can truly prevent that is by minimising the use of such google services.