Bought Wrong Bike: What To Do Now?
Here is another thing to do now: think about what sort of riding you want to do. The Scott Scale appears to be a performance-oriented hardtail MTB. I am not familiar with MTB disciplines, but it may be a cross country (XC) race bike. It is slow on pavement because it is designed for a completely different environment.
If you are mainly on paved roads or trails, there is little need for front suspension. Your tires can provide adequate suspension. Also, drop handlebars offer a better riding experience for most because they give you a large number of potential hand positions. And you don't need knobby tires for traction on paved roads. Actually, while it might seem alarming at first, you can run slick tires on dirt roads. It's only when you start to get a lot of loose gravel or mud that tire tread starts to become meaningful.
If you are mainly riding technical MTB trails, then you need an MTB, or possibly a more MTB-oriented gravel bike. For this type of riding, flat bars do offer advantages. Admittedly, because this is not my preferred type of cycling, I can't speak personally to this. My understanding is that they offer easier and/or more ergonomic braking on steep and/or extended off-road descents. Also, among MTBs, there is a spectrum of capability, with XC bikes being mainly oriented towards light weight and quick handling, trail and all-mountain handling better on descents than XC bikes, and a few more types besides. As I am not current on modern MTBs, I can't tell if enduro bikes just have more suspension travel and descend better than all-mountain bikes, or if there are qualitative differences as well. While this does not seem to fit the OP's use case, a prospective rider who is more oriented toward MTBs might want to do some exploration into the types of riding they expect to do and to enjoy.
If you are riding on general unpaved terrain, it may have a mix of more straight and open dirt or gravel roads and more technical terrain. At this point, the mix will influence your bike choice between a gravel bike and an MTB. There's also personal preference. A lot of people will prefer hardtail MTBs. However, suspension may or may not be strictly necessary to enjoy the ride. In addition, gravel bikes can be used on paved and unpaved roads alike; you may wish to change to slick tires if you are mostly on the road, but even knobby gravel tires roll faster on pavement than MTB tires.
Be aware that modifying a gravel bike to have flat handlebars will be expensive. You are talking about getting a new bar and shifters, likely a different stem to account for the different reach, and I believe new derailleurs. You then need to re-cable the bike and re-bleed the brakes. Why do you feel you need flat handlebars? Is it due to terrain? Is it due to a well-informed personal preference? If the latter, consider that there are a handful of gravel bikes that are offered in stock flat bar configurations, and I imagine there are some fully rigid MTBs. If it is not an informed preference, I would strongly consider trying out drop bars.
The above answer was given mostly from the point of view of performance-oriented bikes. For the use case mentioned in the comments of another answer, a hybrid bicycle might be worth serious consideration. If one is not comfortable with the riding position of a road bike, a hybrid offers a more upright position and usually thicker tires - which offer more suspension than the tires on many road bikes. I alluded to this above, but for riding on paved trails, I would eschew suspension as it is unnecessary. It would add weight, plus you are paying for the suspension. Without it, you could get a cheaper bike, or a bike with better specs on other important parts.