I think 〜な usage is rather colloquial. It's not in the dictionaries and it's less common than 〜の. We don't usually add adj-na in such cases.

@@ -33 +32,0 @@
-<pos>&adj-na;</pos>

I don't think that xref helps.

@@ -34 +33,0 @@
-<xref type="see" seq="2859234">王道的</xref>

Preference towards の　な or 的な a timeless mystery:
王道的な物語	1031	47.6%
王道の物語	211	9.7%
王道な物語	153	7.1%

王道的なやり方	62	2.9%
王道のやり方	165	7.6%
王道なやり方	42	1.9%

王道的なラブストーリー	25	1.2%
王道のラブストーリー	418	19.3%
王道なラブストーリー	58	2.7%

reverso:
大福のあんこは、王道のつぶあんとおいも餡の2種から選択可能。
You have a choice of 2 anko pastes to fill your daifuku with: the classic red b ean 'tsubuan' and the sweet potato 'imoan'.

王道の演出で
In classic storytelling style,

王道なRPGをスマートフォンで実現
Players can enjoy on smartphones a typical RPG

Apparently this is a calque from English(at least for sense [2]). Never heard of it, but "royal road" is the most common reverso gloss.

sankoku does not identify as adj-na, but usage suggests otherwise.  There is a 的 form, which I have created separately. Yourei has 940 examples, 60 of which are 的. の seems more common as an adj. than な, but な does appear.  As an adjective this *seems* to be at least primarily (perhaps entirely) sense [3]. 
https://yourei.jp/王道

We don't commonly xref the 的 forms from the main forms, but at least this offers some [adj] glosses.  This is far less "stodgy" of a usage than I expected. 

Is 王道のやり方 an easy method, or a traditional method?  From what I've seen so far, it seems to be the latter.

@@ -31,0 +32,3 @@
+<pos>&adj-no;</pos>
+<pos>&adj-na;</pos>
+<xref type="see" seq="2859234">王道的</xref>

Ok. I've just reindexed the sentences.

Yes, those sentences are sense 3. Given that it's a newer meaning, I'm not sure about moving it to the top.