Get all fields for a content type using REST
The /entity/entity_form_display/{entity_form_display} should works.

Example: 

/entity/entity_form_display/node.article.default?_format=json

It will return json like this:

{
"uuid": "058dd73d-366d-466d-9393-ad0d5d44e928",
"langcode": "en",
"status": true,
"dependencies": {
    "config": [
        "field.field.node.article.body",
        "field.field.node.article.comment",
        "field.field.node.article.field_image",
        "field.field.node.article.field_location",
        "field.field.node.article.field_tags",
        "image.style.thumbnail",
        "node.type.article"
    ],
    "module": [
        "comment",
        "image",
        "path",
        "text"
    ]
},
"id": "node.article.default",
"targetEntityType": "node",
"bundle": "article",
"mode": "default",
"content": {
    "body": {
        "type": "text_textarea_with_summary",
        "weight": 1,
        "region": "content",
        "settings": {
            "rows": 9,
            "summary_rows": 3,
            "placeholder": ""
        },
        "third_party_settings": []
    },
    "comment": {
        "type": "comment_default",
        "weight": 20,
        "region": "content",
        "settings": [],
        "third_party_settings": []
    },
    "created": {
        "type": "datetime_timestamp",
        "weight": 10,
        "region": "content",
        "settings": [],
        "third_party_settings": []
    },
    "field_image": {
        "type": "image_image",
        "weight": 4,
        "region": "content",
        "settings": {
            "progress_indicator": "throbber",
            "preview_image_style": "thumbnail"
        },
        "third_party_settings": []
    },
    "field_location": {
        "weight": 121,
        "settings": [],
        "third_party_settings": [],
        "type": "options_select",
        "region": "content"
    },
    "field_tags": {
        "type": "entity_reference_autocomplete_tags",
        "weight": 3,
        "region": "content",
        "settings": {
            "match_operator": "CONTAINS",
            "size": 60,
            "placeholder": ""
        },
        "third_party_settings": []
    },
    "path": {
        "type": "path",
        "weight": 30,
        "region": "content",
        "settings": [],
        "third_party_settings": []
    },
    "promote": {
        "type": "boolean_checkbox",
        "settings": {
            "display_label": true
        },
        "weight": 15,
        "region": "content",
        "third_party_settings": []
    },
    "status": {
        "type": "boolean_checkbox",
        "settings": {
            "display_label": true
        },
        "weight": 120,
        "region": "content",
        "third_party_settings": []
    },
    "sticky": {
        "type": "boolean_checkbox",
        "settings": {
            "display_label": true
        },
        "weight": 16,
        "region": "content",
        "third_party_settings": []
    },
    "title": {
        "type": "string_textfield",
        "weight": 0,
        "region": "content",
        "settings": {
            "size": 60,
            "placeholder": ""
        },
        "third_party_settings": []
    },
    "uid": {
        "type": "entity_reference_autocomplete",
        "weight": 5,
        "region": "content",
        "settings": {
            "match_operator": "CONTAINS",
            "size": 60,
            "placeholder": ""
        },
        "third_party_settings": []
    }
},
"hidden": []
}