As per the requirement, we can add dynamic payment methods list with multi select in the system configuration in admin.

For this, first we need to create system.xml file – it is located in directory Magemonkeys/Payment/etc/adminhtml/system.xml where we need to add below code.

<field id="payment_methods" translate="label" sortOrder="20" showInDefault="1" showInWebsite="1"
                       showInStore="1">
    <label>Sku</label>
    <frontend_model>MagemonkeysPaymentBlockSystemConfigFormFieldFields</frontend_model>
    <backend_model>MagentoConfigModelConfigBackendSerializedArraySerialized</backend_model>
    <comment>Dynamic Payment methods list</comment>
</field>

Second thing we need to do is to create Magemonkeys/Payment/Block/System/Config/Form/Field/Fields.php to render multi select field in system configuration.

<?php
declare(strict_types=1);
namespace MagemonkeysPaymentBlockSystemConfigFormField;

use MagentoConfigBlockSystemConfigFormFieldFieldArrayAbstractFieldArray;
use MagemonkeysPaymentBlockAdminhtmlFormFieldPaymentSkuColumn;
use MagentoFrameworkDataObject;
use MagentoFrameworkExceptionLocalizedException;

class Fields extends AbstractFieldArray
{    
    /**
     * @var bool
     */
    protected $_addAfter = false;

    /**
     * @var
     */
    protected $_addButtonLabel;

    /**
     * Rows cache
     *
     * @var array|null
     */
    private $_arrayRowsCache;

    /**
     * @var TaxColumn
     */
    private $paymentRenderer;

    /**
     * Construct
     */
    protected function _construct()
    {
        parent::_construct();
        $this->_addButtonLabel = __('Add');
    }

    /**
     * Prepare to render the columns
     */
    protected function _prepareToRender()
    {
        $label = 'label';
        $this->addColumn('sku', [$label => __('Sku')]);
        $this->addColumn('payment_method', [
            'label' => __('Payment Method'),
            'renderer' => $this->getPaymentRenderer(),
            'extra_params' => 'multiple="multiple"'
        ]);
        $this->_addAfter = false;
        $this->_addButtonLabel = __('Add');
    }

    

    /**
     * Prepare existing row data object
     *
     * @param DataObject $row
     * @throws LocalizedException
     */
    protected function _prepareArrayRow(DataObject $row): void
    {
        $options = [];
        $payment = $row->getPaymentMethod();
        if ($payment !== null) {
            foreach ($payment as $country) {
                $options['option_' . $this->getPaymentRenderer()->calcOptionHash($country)] = 'selected="selected"';
            }
        }

        $row->setData('option_extra_attrs', $options);
    }
    /**
     * @return TaxColumn
     * @throws LocalizedException
     */
    private function getPaymentRenderer()
    {
        if (!$this->paymentRenderer) {
            $this->paymentRenderer = $this->getLayout()->createBlock(
                PaymentSkuColumn::class,
                '',
                ['data' => ['is_render_to_js_template' => true]]
            );
        }
        return $this->paymentRenderer;
    }
}

Then, we need to create Magemonkeys/Payment/Block/Adminhtml/Form/Field/PaymentSkuColumn.php where file returns payment methods list.

<?php
declare(strict_types=1);
namespace MagemonkeysPaymentBlockAdminhtmlFormField;

use MagentoPaymentHelperData;
use MagentoFrameworkViewElementContext;
use MagentoFrameworkViewElementHtmlSelect;
 
class PaymentSkuColumn extends Select
{
    private $paymentHelper;
 
    public function __construct(
        Context $context,
        Data $paymentHelper,
        array $data = []
    ) {
        parent::__construct($context, $data);
        $this->paymentHelper = $paymentHelper;
    }
 
    public function setInputName($value)
    {
        return $this->setName($value . '[]');
    }
 
    public function _toHtml(): string
    {
        $options = [];
        $paymentMethod = $this->paymentHelper->getPaymentMethods();
        foreach ($paymentMethod as $key => $payment) {
            if(isset($payment['title'])){
                $options[] = ['label' => $payment['title'], 'value' => $key];
            }else{
                $options[] = ['label' => $key, 'value' => $key];
            }   
        }

        if (!$this->getOptions()) {
            $this->setOptions($options);
        }
        $this->setExtraParams('multiple="multiple"');
        return parent::_toHtml();
    }
}

It will look something like this.