What happened to my Betta?

Hi all,
My Betta passed away after an illness yesterday morning. I'm trying to find answers and simply can't find anything that solves the puzzle for me. He was with me for 2.5 years, so estimated 3-3.5 years old. My heart is completely shattered, and the worst part is I don't know what could have caused this.

Water parameters have been fine this whole time, 80l/20G tank is planted with live plants, filtered, heated to 28c, and had 2 mystery snail guests with him the whole time. Weekly 25% water changes to keep everything fresh. He had a diet of Bug Bites flakes, Hikari Bio-Gold & Northfin pellets.

Timeline:

• Start - Friday night - Ate normally (if slightly too much food).
• Saturday - Noticed a bloated, swollen belly, attributed to too much food. Water change, fast day, no improvement.
• Three day fast - no improvement. Still bright, alert, active & eating. Happily ate frozen Daphnia.
• Week 1 passed - no change in belly, still bright/alert/hungry.
• Week 2 passed uneventfully. Still no changes in belly, still alert/hungry.
• Week 2 - my previous Reddit post was made (with images). Still large bloated belly, still bright, alert and hungry. No behaviour changes at all and he seemed in perfect health apart from the belly. Another 2 day fast to see if anything changed. Unfortunately no ideas or comments made to give me any direction to work with, and he seemed okay aside from the belly.
• Week 2/3 Saturday - another water change.
• Week 3 Sunday - passed a very very large poop (the length of his body)
• Week 3 Monday - second body length poop. Still active, bright, normal. Belly still large and unchanging.
• Week 3 Friday night - Hiding and lethargic, still ate pellets but wanted to hide a bit.
• Week 4 Saturday - Water change day. Back to normal behaviour, happily greeting and interacting. Ate lightly but nothing to a point of concern.
• Week 4 Sunday morning - Seemed slightly tilted/lopsided, belly still the same. Refused food, wanted to rest more.
• Week 4 Sunday night - Began swimming erratically/corkscrewing, then stopping and floating on an angle gasping. Began losing control after these episodes, ending up on his side over decorations. Seemed minimally responsive. Head started twitching non-stop. Noted very slight pineconing only on the swollen parts of the belly. Started looking into euthanasia.
• Week 4 Monday morning (very early hours) - Woke up to find him deceased.

What could this be? Any ideas?