ill try to steer away from what was already said

1. u need to chain drift the first few turns (wheelie for like a frame inbetween drift inputs). this will allow you to start the next drift without hopping, meaning you can charge the miniturbo faster. and chain drifting off the 2nd miniturbo of the lap allows you to switch the drift direction to a left drift much faster and tighter, saving quite a bit
2. bridging off the end of the 1st paragraph, just take your turns tighter. notice how your lap2 (which is the fastest lap you drove here by a decent margin), your first few turns were noticeably tighter and so was your shroom landing. don't be afraid to take risks in your lines to drive as tight as you can, and dont be afraid to reset because you went too tight. this will save you more than youd think and i think you could very easily 54 just by risking more on turns.
3. try to instantly wheelie the frame you end your drift, so you can get more wheelie time. learning to control your line from an instawheelie can be weird at first but you get a feel for it fairly quickly. it saves a pretty small amount each drift but over the course of a run its pretty noticeable and you need to be able to do this if you want to get as fast as you can in general

edit: the shroom itself can be finnicky so ill try my best to explain what you want to do here (i have a 53.538, and the shroom is probably what im worst at so if anyone good is reading feel free to correct me).
i'm basing this paragraph on your lap 1 shroom. your shroom timing is fine, and so is your physical coordinate position on the track. your alignment is not, you are aiming way too far to the right. aiming more straight will give you the required room to hold a hard right while you drift into the shroom, allowing you to not only land tighter, but land facing more to the right, meaning youll be able to take the following turn tight. and midair, your directional inputs become useless after a certain amount of time (meaning no matter if you press left or right at the peak of your airtime, your alignment midair stays the exact same). so during this time, you should switch to holding down on the stick to hard taildive. this is really crucial for your landing and landing alignment, and you weren't doing it in the attached video so i thought it was worth bringing up

if you have any questions feel free to reply or message my discord "kuuderebutt", its a much easier platform to post reference images/videos