
Bose
SoundLink Flex Series
Compact, durable, impressive sound for size, but distorts high volume.

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Jbl partybox encore essential if youre on a budget. Just keep it plugged when in use as the battery is dog water. Sound quality and loudness is unmatched for its size though.
OP, Ignore some of the comments suggesting small sized BT speakers for outdoor usage. Go for the JBL Partybox Encore Essential with eyes closed, it's available under 15k on Flipkart/Amazon.
I just bought the JBL party box essential 79.99 at ross
jbl encore essential 2 would do the job and its splash safe
for the price you cannot beat the partybox 120, relatively light and small ish, battery is amazing and the bass and even sound clarity is best in class. i never need to use it with max volume as i mainly use it indoors, however if portability is an issue, get the encore essential 2, honesty with deeper bass it keeps up really well. if portability is not really a massive issue, but still keeping things at a decent price get either 2x 320s or one 520. yes they are bigger and heavier, but worth it imo. i have owned 2x 320s since launch and they have never missed a beat, it punches wayy harder at higher volumes and when plugged in, it will rattle your house, but if price is not an issue get the partybox 720, the bass is unimaginable, the clarity is still good even though there is a lack of midrange drivers, but the mids are well kept by JBL’s best in class DSP, or what they call “Ai sound boost”
Don't think you're "missing" anything... The PB OTG ESS just isn't an amazing speaker-- I've heard most of JBL's lineup and the Encore / Encore Essential are much better. IIRC according to FCC filings the OTG is actually built with "off the shelf" parts from Eastech while I believe most other JBL's have somewhat custom parts.
If those are your only options, go with either one of the Encores but none of these will be ideal for filling such a large space with sound. For reference the Partybox120 would be the minimum I'd reccomend. The XTREME4 is not a bad speaker but it won't be able to fill the space and it sounds very sharp and compressed at high volumes-- not exactly a pleasant sound especially for bass-heavy music. The Encores get slightly louder IIRC and sound better at high volumes (given the area you're looking to fill, you'll probably be using them close to max volume). If you don't need the microphones or the easily replaceable battery, you can save some money and go with the Encore Essential 2. Alternatively, I think a portable PA speaker may be better suited for your use case. A small portable PA like the Alto Busker or Mackie Thump Go will likely get louder than the Encores while still retaining the portability sounding good enough. And if you have more room in your budget check out the Bose S1 Pro or JBL EON ONE Compact for even better sound.
Please note that the PARTYBOX is in a completely different category compared to the CHARGE5. If you're looking to **fill a large room or outdoor space with loud sound & bass, I'd definitely go for the PARTYBOX (JBL advertises the PARTYBOX to fill a tennis-court sized space indoors or outdoors)** especially since you already have UE BOOM for portable usage. The CHARGE5 is going to be an excellent & noticeable upgrade to your UE BOOM (and so will the CHARGE6, also going for \~30% off depending on location) but it's still a medium-sized portable speaker-- as far as I'm aware, there's no moores law or anything of the sort in the speaker industry: we've generally been getting & using the same principles to build transducers for nearly a century now. If you're looking for a good in-between option, the new BOOMBOX4 gets just as loud as the PARTYBOX CLUB 120 (albiet with some noticeable distortion on select tracks) but lacks the mic/guitar inputs: in exchange you get much better durability (IP68 vs IPX4).

Bose
SoundLink Flex Series
Compact, durable, impressive sound for size, but distorts high volume.

Minirig
Minirig 4
Tiny, unrivaled sound for DJs, but pricey with bad Bluetooth.

JBL
Charge 6
Portable, durable, punchy bass, but distorts up close, dull vocals.

soundcore
Boom 2 Plus
Budget king for loud, deep bass; bulky with harsh mids.

Anker
Select 4 Go
Great value, good stereo paired, but lacks power for parties.

Ranked #1
Anker - Select 4 Go

Ranked #1
Harman Kardon - Go + Play 3

Ranked #1
LG Electronics - XBOOM Grab

Ranked #1
soundcore - Boom 3i

Ranked #1
JBL - PartyBox 520

Ranked #1
Bose - SoundLink Flex Series