Well, I think I found a solution to this in the most Gutenberg way possible.
Set block template javascript variable in the template, then read that variable instead of relying on wp.data.select(“core/block-editor”).
Since Gutenberg is all about saving the data into the html this seemed like the best way to get field data from other blocks.
<?php
// Dynamic block ID.
$block_id = 'repeater-' . $block['id'];
$blockClasses = implode( ' ', array( $block['className'] ) );
$per_page = get_field( 'per_page' );
$allowed_blocks = array( 'acf/card' );
?>
<div id="<?php echo $block_id; ?>" class="<?php echo $blockClasses; ?>">
<InnerBlocks templateLock="false" allowedBlocks="<?php echo esc_attr( wp_json_encode( $allowed_blocks ) ); ?>"/>
</div>
<div class="items-repeat"></div>
<script>
var <?php echo $block['id']; ?> = {
'per_page': <?php echo $per_page; ?>
}
</script>
Then in my javascript onchange I can get the block / field from the variable
window[block.attributes.id].per_page
