From the perspective of background, it's simply not pleasing to look at I'm afraid to say, as there are implausible unit combinations (Harlequins are never commanded by non-Harlequin characters, and do not use craftworld transports, for example), and the presence of two Phoenix Lords in anything calling itself a regular Eldar army list (and that's what this is, since this is not a list for a narrative campaign, or an Apocalypse battle) is exceptionally unlikely. Of course, if this does not bother you, then feel free to ignore this, but as you are someone who has taken an interest in background in the past, I just thought that I would draw this to your attention
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In terms of effectiveness, I am also very unconvinced about the balance of the army list, because you have skewed the army too far in favour of the mechanised component at the expense of the infantry section, which is just too small, and too poorly protected to withstand any sort of attack in my view.
The Guardians are effectively a filler unit, which is always a bad idea in Eldar armies in my opinion, while Dark Reapers work much better when attached to substantial fire bases in my experience, as if left alone, they are too easily neutralised by units which can outflank or deep strike. I thus recommend that you invest more points in the infantry section, or drop it completely, and field a fully mechanised list.
Taking Harlequins in this list is a bad idea all round in my view. Not only does the background look questionable, but also not being able to move flat out in the first turn, due to not being able to start on board the Wave Serpent, makes them a tactically unsound choice in my view, as they cannot keep up with the rest of the mechanised wing, meaning that your attack ends up being somewhat fragmented.
Giving the Storm Guardian squad's Warlock Enhance does not really help them that much in my experience, so I feel that you would be better off keeping them out of close combat, and giving their Warlock Destructor, so that they can fulfil a dual purpose role.
Finally, I do not recommend giving the Dark Reaper Exarch the EML should you keep the unit. The squad excels at shooting MEQs, nothing else, and if you take the EML for shooting vehicles, you waste the firepower of a very expensive squad, which is not a good tactical move in my view, hence the Tempest Launcher is a better bet in my experience.
Overall, I understand that this is meant to be a fun list, and the mechanised wing is certainly very strong, but it is horribly skewed in terms of the infantry/mechanised balance, and has a whole bunch of points tied up in two models and expensive units, such as Harlequins and Dark Reapers, resulting in a very low model count, and an army which is a massive glass cannon, even by Eldar standards. How the list will perform, I cannot be sure, as it really is not the sort of list I have fielded in fifth edition, but it strikes me as being more of a second edition list, which is unlikely to work all that well under the current rules. I'm sorry if you find that to be a rather negative conclusion, but while you may find yourself doing well with it against certain lists, I envisage it faltering more often than succeeding.
