I think it might be time for another backpack upgrade.

[noobsqoou]

With 1.5 and 1.6 there have been quite a few new additions to the drop pool for fishing, foraging, combat, and mining, and even with both backpack upgrades I’ve been running into unusually frequent instances of having to trash rare items while doing some light fishing, scavenging, or adventuring. As such, I feel it might be time for a third backpack upgrade.

As for when this should be made available, there are a few ideas:

• Purchased from Pierre’s for 50,000g — this would be the most obvious option, but it’s kinda bland
• Traded from the Desert Trader — this would create an interesting opportunity for a unique cost, but it could also put too much emphasis on completing the vault bundle
• Reward for completing the mastery cave — this would have a satisfying thematic feel to it and would coincide with some of the most recent inventory-flooding culprits: trinkets, golden mystery boxes, golden animal crackers, golden fishing treasure chest loot, and extra gems from gem rocks
• Traded from the Island Trader — like the Desert Trader, this would create an opportunity for an interesting cost, but, instead of being locked behind one bundle, it would be locked behind the entire community center (or Joja Warehouse)
• Purchased from the Walnut Room — certainly an option, but I feel this would be too late in the game to matter, as at that point you’re probably trashing everything that isn’t 1000 kegs of starfruit or ancient fruit wine

 

[Call_The_Banners]

My only concern would be tabbing through 4 separate bars instead of 3. Feels like it could get a little clumsy unless there was a way to select the specific action bar.

 

[stardew_luv]

I agree. There’s so much more to collect since 1.5 was released and now 1.6 makes it insane. There’s mods for this but I haven’t had luck with mods working right. And they tend to mess my game up.

 

[Quirinea]

"Item bags" is a mod that I've been using occasionally. It doesn't cause a mess but it's easy to forget what you have put in the bag, as they won't show.

 

[ThePerfectGarden]

I'm absolutely pro - it was a suggestion I thought about to put in here myself.
Out of your propositions, I would reject the first and the last one. The first one would be bland indeed, and Qi's Walnut Room is nothing I would like to support any more (there's enough stressing quests in it).
Desert Trader and Island Trader are okay, but personally the mastery cave speaks to me the most.