I think it's about finding the songs. I did some googling to find suggestions in the beginning. Now I'm thinking a P5 is it don't mean a thing, M3 Swing low chariot, me Morning Mood by Grieg, m6 theme from Love Story, for instance. Still singing the descending intervals backwards in my head sometimes though.

singing the note out helps but it'll take a while until you internalize it. if you can recognize the intervals ascending then just sing it ascending after listening to the descending one

I try to remember the first note, listen the second one, and then sing the one I remember. Maybe sound kinda cheating, but you still need to remember, thing, listen in your head, etc., is not worthless if you struggle with this.

Hum them back in reverse order so that now it's an ascending interval.

But a specific mnemonic for the descending major third, at least, would be the doorbell sound (ding-dong).

Yea I try to guess intuitively first, and then reverse it and sing it back to check my guess lol

I use the theme from The A-Team for the descending 4th and The Flintstones theme for the descending 5th

Thanks guys! I have tried humming them in reverse and it works a bit, but doesnt really help me internalise the sound at its face value. I have worked out a few recognisable sounds now, from sections of different star wars melodies again ha ha ha. And minor 3rd became pretty obvious to a few songs after a while. Still hard, but definitely made in roads!
Thanks!

I apologize for the capital letters, but I'm pasting this across from my notes elsewhere and I don't want to put them all in lower-case now. Some of the songs, such as 'Shave and a Haircut' or the theme to 'Love Story' are probably very familiar to you but you just didn't know their names.

MINOR 2ND
FÜR ELISE

MAJOR 2ND
THREE BLIND MICE (two major thirds)

MINOR 3RD
MAD WORLD (up and then down a minor third) / STAR-SPANGLED BANNER / HEY JUDE / FROSTY, THE SNOWMAN (the first two notes of these last three)

MAJOR 3RD
START OF BEETHOVEN’S FIFTH / SWING LOW, SWEET CHARIOT / CLASSIC DOORBELL SOUND

PERFECT 4TH
SHAVE AND A HAIRCUT

TRITONE
EMERGENCY VEHICLE SIREN

PERFECT 5TH
THE FLINTSTONES

MINOR 6TH
THEME FROM LOVE STORY (this goes down and then up a minor sixth at the start)

MAJOR 6TH
NOBODY KNOWS THE TROUBLE I’VE SEEN (a great song by Louis Armstrong that you should familiarize yourself with, anyway! The first two notes of the melody are dropping a major sixth)

MINOR 7TH
WATERMELON MAN’; 'CHRIST - MAS -ES’ DROP AT THE END OF WHITE CHRISTMAS

MAJOR 7TH
NONE I CAN THINK OF

This looks interesting. https://www.musicca.com/interval-song-chart#:~:text=1.%20Ascending%20intervals%20%20%20%20Interval%20,1929%29%20%20...%20%209%20more%20rows%20

That link is superb. The first two notes Eddie Vedder sings in Even Flow: a descending tritone. Great to know :-) And No Surprises and Man in the Mirror for the descending major sixth, too.

Have you tried the interval memorizer tool?
https://www.tonegym.co/tool/item?id=interval-memorizer