Add new variables in mail template

Good day!
3 new items need to be added to the Shopware basic letter, by the request of the client.
The first 2 are custom fields that are listed in the product itself. The third value is one of the properties of the product.
Version of the Shopware 6.3.5.2. This version has no convenient hints with the selection of variables for letters.
At first I thought that the variable {{ order.lineItems.0.customFields }} would help me find a list of custom fields of the product and its values. But it didn’t work. This also didn’t work {{ lineItem.product.customFields }} .
For property also did not find anything.
I tried to dump these templates, but it didn’t work. Is it possible to do a dump for these emails at all?

I would be grateful for any hint!