I have checked the previous edition of the EU-OPS, and the one before that... still no definition of Suitable Airport...
Then, as you mentioned the EU-OPS 1.192, it makes reference to c/ En-Route Aternate (ERA) and d/ 3%ERA...
What is for you, at the planning stage a C/ EN-ROUTE ALTERNATE ?considering that it is a part of the IFR planning stage, NON ETOPS...
cf definition: en-route alternate (ERA) aerodrome’ means an adequate aerodrome along the route, which may be required at the planning stage;