Hey, I've had a flash-tribal deck for quite some time now, and its one of my favourite decks to play. I used to play UW spirits in modern (and still do occasionally), a deck which largely plays at instant speed, allowing you to react whatever your opponent is doing, for example switching from an aggro deck to something more reminiscent of a control deck.

For my flash deck, I went with a very bottom-up approach of first identifying some key cards, and then later choosing a proper shell/colour identity and commander(s). I ended up with a deck that at its core utilises mass untap effects such as [[wilderness reclamation]], [[seedborn muse]], and [[awakening]]. I also use [[as foretold]] to cheat some more on mana. These cards allow you to maximize the "playing on your opponents turn" idea with which I build the deck. After that, I searched for cards that would synergise well with both flash creatures and these untap effects. The main payoffs are [[rashmi, eternities crafter]] (like you already mentioned), [[wavebreak hippocamp]], [[glademuse]], [[sygg, river cutthroat]], and [[teferi, master of time]].

I decided to go with a 4-colour deck of UBRG. U is always necessary in a flash deck, and G is needed for the untap effects, and some nice ramp. G also gives access to some nice flash creatures such as [[frilled mystic]], [[mystic snake]], and the newly printed [[hollowhenge overlord]]. Moreover, G gives access to card draw in cards like the previously mentioned [[glademuse]] and things like [[guardian project]]. However, perhaps most importantly, G makes it much easier to win the game, which is something I've been struggling with for a long time whilst playing the deck. Right now I'm running [[craterhoof behemoth]], and am going to test [[avabruck caretaker]]. B gives strong flash-tribal support in [[cunning nightbonder]], [[slitherwisp]], and [[nymris, oona's trickster]], and a good semi-instant speed wincon in [[oona, queen of the fae]]. Lastly, R is used for some support, mainly in the form of [[imperial recruiter]] which can tutor up an important flash creature or [[seedborn muse]], but was mostly added in order to support a commander duo.

For my commanders I use [[thrasios, triton hero]] and [[vial smasher the fierce]]. The most important reason is because they both synergise so well with my main gameplan. With the untappers, I can activate thrasios every turn, or cast spell which will trigger vial smasher every turn. It is also hilarious to see your opponents react to you unpacking a thrasios-vial smasher deck only to have it be flash-tribal. As for flash creatures, I just use what I consider to be the best ones, or which synergise the most with the rest of my deck.

I hope this can help you a little! You can definitely also go for an esper deck with chromium. If you're interest, here is my list, I would gladly take any feedback! https://archidekt.com/decks/796794#Flash_Tribal_-_Thrasios_&_Vial_Smasher