As you know it from my comments in YT I used DaVinci Resolve before. I'm not fluent (just amator making family movies from holidays, family events etc.). In fact I don't use many of the efects but one I used before in DaVR was speeding or slowing clips in the smooth way. I can't figure out how to change speed of the part of the clip in smooth way in HitFilm. You have showed it in your tutorial for beginners how to change the speed of clip but I still don't know if it is possible to change speed in the smooth way using keyframes and curves like I did it in DaVR. Is it possible in HitFilm (I presume it is) ?
You can actually achieve that in HitFilm using the 'Speed' effect. The effect is also available in HitFilm Express for free Simply apply the effect to your layer within a Composite Shot and then you can keyframe the 'Speed' property to you needs to smoothly ramp the playback speed up or down.
You can also use the Curves editor if you need more control over the exact value change between your keyframes. Hope that helps!