Custom table to WordPress schema migration base class proof of concept. NOT TESTED.

class-migrate.php

<?php
/**
 * Migration Abstract class.
 *
 * Should inherit this class and all documented class variable must overwrite in child class for this to work.
 *
 * @package rtCamp
 */

namespace rtCamp\TableMigrate;

abstract class Migrate {

	/**
	 * Table to migrate.
	 *
	 * @type string
	 */
	const TABLE_NAME = '';

	public $total_records;

	public $limit = 100;

	public $migrated_records = 0;

	public $current_row;

	/**
	 * Post type name.
	 *
	 * @var string
	 */
	public $post_type;

	/**
	 * Taxonomy name.
	 *
	 * @var string
	 */
	public $tax;

	/**
	 * Mapping should have wp_insert / wp_term key and mapping value from table column.
	 *
	 * example:
	 * [
	 * 'post_title' => 'column_name',
	 * 'post_status' => ['name' => 'column_name', 'callback' => {callback function}]
	 * 'meta_input' =>  [
	 *   'meta_key' => 'column_name',
	 *   'meta_key2' => ['name' => 'col_name', 'callback' => {callback function} ]
	 * ] ]
	 *
	 * @var array
	 */
	public $mapping = [];

	/**
	 * Is CPT migration.
	 *
	 * @var bool
	 */
	public $is_cpt;

	/**
	 * Is taxonomy migration.
	 *
	 * @var bool
	 */
	public $is_tax;

	public function __construct() {
		if ( $this->is_mapping_exists() ) {
			echo 'Mapping can\'t be empty in' . get_class( $this );
			die;
		}
		$this->total_records = intval( $this->get_total_records() );
	}

	protected function is_mapping_exists() {
		return ! empty( $this->mapping );
	}

	public function start() {
		if ( $this->migrated_records === $this->total_records ) { // Migration complete.
			return true;
		}

		while ( $this->migrated_records !== $this->total_records ) {
			$data = $this->get_next_records();

			foreach ( $data as $row ) {
				$this->current_row = $row;
				$wp_data           = $this->map_columns( $this->mapping, $row );

				// Insert data.
				if ( $this->is_cpt ) {
					$wp_data['post_type'] = $this->post_type;
					wp_insert_post( $wp_data );
				} elseif ( $this->is_tax ) {
					$term      = $wp_data['term'];
					$term_meta = $wp_data['term_meta'];
					unset( $wp_data['term'], $wp_data['term_meta'] );
					$term_id = wp_insert_term( $term, $this->tax, $wp_data );
					foreach ( $term_meta as $key => $val ) {
						add_term_meta( $term_id['term_id'], $key, $val );
					}
				}
				$this->migrated_records ++;
			}
		}
		$this->current_row = null;
	}

	public function map_columns( $mapping, $row ) {
		$wp_data = [];
		foreach ( $mapping as $wp_col => $mapping_table_col ) {
			if ( is_array( $mapping_table_col )
			     &&
			     array_key_exists( 'name', $mapping_table_col )
			     &&
			     array_key_exists( 'callback', $mapping_table_col )
			     &&
			     is_callable( $mapping_table_col['callback'] ) ) {
				$wp_data[ $wp_col ] = call_user_func( $mapping_table_col['callback'], $row[ $mapping_table_col['name'] ], $mapping_table_col['name'], $row, $this );
			} else if ( is_array( $mapping_table_col ) ) {
				// In case of array of array for post_meta and term_meta.
				$wp_data[ $wp_col ] = $this->map_columns( $mapping_table_col, $row );
			} else {
				$wp_data[ $wp_col ] = $row[ $mapping_table_col ];
			}
		}

		return $wp_data;
	}

	public function get_query() {
		return sprintf( 'SELECT * FROM %s LIMIT %u, %u', static::TABLE_NAME, $this->migrated_records, $this->limit );
	}

	public function get_total_records() {
		global $wpdb;

		$sql = sprintf( 'SELECT COUNT(*) FROM %s', static::TABLE_NAME );

		return $wpdb->get_var( $sql );
	}

	public function get_next_records() {
		global $wpdb;

		$sql = $this->get_query();

		return $wpdb->get_results( $sql, ARRAY_A ); // Returns array.
	}
}