Here's my similar experience and a bunch of tips that might help you.
I have been doing over 4min for a few months with a pb of 4.35min back in April when i did a max attempt and had a big samba (some guys called it BO) at just about 5min mark, which was my aim. Wrong. A 30sec increase is too much if you do max attempts regularely, as i used to. After the samba i stopped doing max attempts for almost two months, call it a bit of fear but also trying a different approach to go maximum only once in a while when i feel ready for it.
In June, after several easy 4.30min to 4.45min i went again for the 5 and did it with some room to spare. I found the mental reward the greatest, once you know it's done (be it 5, 6 or whatever minutes) doing a 4,30min now feels much easier. I think we set our minds to arbitrary limits and this is somewhat detrimental to performance. Another proof is timing yourself. i used to do it all the time and that was the only way (i thought) to reach a desired time. Just receintly I was forced not to have a timer as somebody else was timing me and it was so much easier to reach very good times that now i am thinking to use a timer only for max attempts and only when somebody else is timing me.
To conclude, I suggest you to stop timing yourself (unless doing tables) for a while (2-3 weeks) - dont cheat

- then have somebody as safety (which you should anyway) to time you and also to give you signals or talk to you on the last part of the attempt - it can work miracles.
Finally, i do max attempts (and generally statics) only in the morning, on empty stomach. As for the breath up routine, i see no difference in my performance between packing and full inhale, i hear that only at top performers it may add a 30sec, if done properly but at our level i bet the effort of packing and holding the extra air is eating up all the additional oxygen reserves you pack in. So , i dont pack any longer, i think the way forward beyond 5min, in my case, is to increase the period of full relaxation before the first contractions come in and that is done certainly without packing.
good luck hitting the 5s!
serge