I was dragged here by the word 'Indonesia', but since the word also start an out of topic discussion, let's just leave Indonesia to me, lol.
Best long haul aircraft doesn't necessarily a wide body aircraft. By AS classification, long haul is route longer than 5000km as depicted in your route network map in your airline info page. Any aircraft capable of flying more than 5000km with maximum payload should not be overlooked. Coming from a reduced demand gameworld and a country with more domestic traffic than international traffic, I need to be careful with my long haul aircraft selection.
So my two personal favorite: Boeing 737-900ER HGW variants for long haul and Boeing 737-700 ER variants for ultra long haul.
It's a courtesy to use OP's hub for an example rather than own hub, so let's try TUN. Within 5000km range, you already cover all Middle East Mega hubs: DXB, DOH, AUH, JED, RUH, so I supposed you want to fly longer than that although so I far I only saw you sent 5x weekly 777 to DXB, 2x weekly 777 to RUH and ERJ to JED.
I'm not familiar with demand in Devau, but since you already have Boeing 737 in your maintenance list, why don't use the same maintenance type to test the water. If I were you, I would send some 737-700 ER to fly TUN-JFK/EWR/YYZ/BOS and some 737-900ER to fly TUN-DXB/DOH/DEL/BOM/AUH/NBO. With the right configuration and proper wave time, matching your short and medium haul routes, I'm confidence your long haul route will work. TUN has 60% empty slots so flying two narrow body back to back is better than one wide body, IRL business customer prefer frequency and in AS demand distribution also favor more frequency.
Why a narrow body vs a wide body? We saw the trend in real world, with airline using remaining Boeing 757 to fly over the pond, and how popular the Airbus A321-200 NEO LR, selling like hot cakes and dubbed the replacement for Boeing 757. So a capable narrow body should make a good long haul aircraft, lower density configuration and other on-board services and amenities might help minimize the perception of less inconvenient narrow-body for longer routes. BA 1 LCY-JFK a 5,500km route is flown on Airbus A318, (albeit with a stop in SNN for US pre-clearance), featuring all business class.
So I think the logic and the reality is there, for the narrow body to shine on longer routes. Of course, it doesn't hurt if you could snatch a wide-body for a very cheap lease price in used market, but still it's more expensive to fly as it consumer more fuel and add another maintenance type.