Blackplayer EX (by FifthSource) is by far the best music player that I have ever used.
It is a paid app,but has a free version to try.
It has a very high level of customization and features.
The team behind it seem to innovate much more than most and really keep polishing it up and trying their best to overcome the many hurdles and limitations that Google has with their music player setup.
I like it because it is great looking and is very intuitive to use.
Important to me in a Music Player are
1) Flac support/ great sound
2) Easy to bounce from Playlist/Genre/Artist pages.
3) Has an easy to use Playing now Que.
4) Is a nice looking app, with great looking playing now
5) Chromecast support
6) Has a good EQ
I FLAC players I have used:
PlayerPro Music Player (I liked it and used it for years but it looks very dated now and has fallen off in features)
Pulsar+ (clean and neat - think it is just almost a re-skin of the standard Google MP3 player.
Pi Music player (seems to be a re-skin as well, as are MANY of the new players out there)
Onkyo HF Player (was OK)
Foobar 2000 - (could not get to work)
AIMP (Awful)
JetAudio HD Music Player (Awful)
n7player Music PLayer (Awful)
Stellio Player (Awful)
(most players are awful)
THAT BEING SAID...
The player will be less important to the sound than how you transmit the music.
To get the very best out of flac files to be sent via Bluetooth, you should try one of the two new codecs included into the new builds of Android and included in many newer Android phones.
Qualcomm developed AptX HD
Sony developed LDAC.
Support is built into Oreo, but phone makers don't have to support them.
Both are said to use a brand new method of 'folding over' of certain frequencies into lesser congested ones.
This is an awful example of what the codecs do, but after reading a few articles and papers on them it is the best I can do in a sentence.
It is totally different than the psychoacoustic model used in MP3 and almost everything else.
It is said to be vastly superior and capable of delivering HD (better than CD) over bluetooth when using HD sources.
The standard Codec for bluetooth is SBC and it is awful.
Apple pushes ACC for better better audio and Qualcomm bought AptX and pushed it for the same.
AptX was the best and was said to be able to get close to 'CD quality' sound but came up short in many uses.
While both AptX HD and LDAC are both 'lossy' codecs, they are more efficient and use more bandwidth to send the data.
So... they better sound.
They are both said to be able to send a file that is audibly the same as a HD (24 bit- better than CD quality)
LDAC supports higher bit rates (24bit/94khz) but adjusts everything on the fly (lowers resolution) to maintain a strong signal.
AptX HD 'only' supports (24 bit/48khz)but is supposed to maintain a more constant average in resolution.
Now, nothing truly matters much if you are streaming 128K Pop music files to a cheap bluetooth speaker or set of earbuds.
But, you are using lossless FLAC and sending to a very well reviewed set of Sony 1000XM3 headphones.
The headphones support both LDAC and AptX HD along with regular AptX.
Sadly Samsung has been kind of a turd when it comes to audio.
The S9 and new Note series does support LDAC, but it left out Apt X HD.
Sadly your Note 4 does not support either.
I think you should get a new phone.
This is based on the assumption that you are choosing FLAC because you hear a difference between it and MP3 (some people can't) and want the better quality.
You got the top rated NC BT headphone on the planet and it wasn't super cheap.
Even if you are 'only' using 16/44 CD quality FLAC files, you should most likely benefit from the improved codecs.
Many even older phones from LG, Oneplus, Sony, and HTC, do support both of the new codecs.
I just got a LG G7 yesterday and am very eager to try the new BT codecs soon, the Quad DAC makes my wired headphones sound very nice!
Since you have the wonderfully reviewed Sony and are using FLAC, you should give LDAC or AptX HD a try.
One really good source article regarding these codecs
https://www.audioholics.com/audio-technologies/bluetooth-audio-guide