Add this CSS:
@media(min-width: 769px) {
:root {
--sidebar-w: 350px;
}
.right-sidebar #right-sidebar {
width: var(--sidebar-w);
}
.right-sidebar .site-content .content-area {
width: calc( 100% - var(--sidebar-w) );
}
}
Change this line: --sidebar-w: 350px;
to set the width of your Sidebar.
Now you can change the Container Width in the Customizer > Layout > Container and the main content will shrink or grow to fill the space, without changing the sidebars width.