So running some numbers here.
You can get roughly ~50k coalescing visions if you go ham and do all of the daily content + assaults. Meaning it should take you about 3 weeks to get cloak to rank 15.
Your alt will start with a -10k echo deficit (assuming you need to buy 4 rank 3 essences).
Luckily those same assaults you were doing reduces that to only -4k (1k echoes 2x per week x3 weeks for the assaults).
Now during this time you were doing visions - let’s assume each run is minimally successful w/o a carry and includes Thrall (granted thrall may not be required for quest but assuming best case).
Rank 1-5 is just first area so 250*5 = 1250 - lowers your essence deficit to -2750 echoes.
Rank 6 is 1 bonus area I believe so 250+125=375 lowers deficit to -2375 echoes.
Rank 7,8 require 2 bonus areas each so (250+125+125)*2=1000 lowers deficit to -1375 echoes.
Rank 9,10,11 each require full clears so 750*3=2250 echoes reducing deficit to 125 echoes.
So you will get your essences for “free” (i.e. just visions + assaults) echo currency at roughly rank ~12. Probably around 10 or 11 if you include heroic darkshore and LFR.
Finishing out the math though
Rank 12, 13, 14, 15 assume full clears (since you need lost areas assuming you can do lower areas and boss) - so 4*750 = 3000 echoes.
In this time, roughly 21 emmisaries will have come and gone so 21*250 = 5,250. I didn’t include this since it isn’t “free” (as in outside of normal cloak upgrade activities) - but still worth noting. This brings total to ~8250 in surplus by the time your done.
This does not take into account any of the other sources, but it also assumes your vision runs will go perfect each time (no failed attempts).
Your mileage will vary on the other sources given that you still actually need to be farming gear throughout all of this (that leg of progression still exists Blizz - just an fyi) and I’m assuming you aren’t just being carried through visions or heroics from day 1.
So basically the first 3 weeks of your alt will be spent spamming dailies/assaults/etc. just to get that cloak to a semi-competitive state. It should leave you with 4 rank 3 essences (minimum needed) and likely at least enough to purchase 1 corruption.
Good luck playing your main during this time though as this assumes going pretty ham on farming currency/running visions.
So is it better than nothing. Sure. Does it leave a lot to be desired still? Absolutely.
Oh by the way, If you didn’t play at all in 8.2 you are still just SoL depending on which essence grinds you need to do - but that is a whole other debacle.