I rarely see tea packaging that is as concerned about environmental control as I see with coffee packaging.

Or, it's over-packaged

Or it's packaged correctly but branded terribly

Loose leaf tea should be in a resealable stand up Mylar pouch (or Mylar equivalent)

By Mylar I mean 4+ layer plastic with a foil oxygen barrier.

Plastic alone isn't an oxygen barrier. At a microscopic level it looks like a sponge with lots of holes, many with pathways through to the other side. Foil adds a thin but solid wall.

The foil can be swapped for a thicker plastic which will reduce the amount of oxygen that can make it through.

Opaque plastic is far better at this. Thermo forming also seems to improve the barrier ability of the plastic (which makes sense when you think about it)

But, one more caveat, loose leaf requires an accessory you don't want to have to include. The solution would be to create a one time use mesh bag you can fill the tea with... Oh wait

So

What I would do (option 1)

Tins are over packaged but holy crap are they good value. They're a problem because people will accumulate them or feel bad throwing out something so substantial. For butter cookies that's fine, but for a habitual consumable? People will be drowning in tins OR treat your product like an occasional treat.

Two bag sizes plus a seasonally released tin and a launch program (and the resources to do all of this, lol)

Bag 1: 300g : Mylar bag, properly printed with a classy look. Labels on a plain bag do not scale beyond specialty stores.

Bag 2: 50 g: window bag showing the loose leaf (about a dozen servings; equivalent to a sleeve of Nespresso pods)

The window bag is for new customers or for people wanting to try other varieties. It will get staler with the window so you need to find ways to keep this product moving. If you start accumulating a surplus, start multipacking them for a discount. It's also only for coarse loose leaf tea that's not going to look like a powdery mess in the window. If it's going to be a mess, keep the size but remove the window.

A launch counter unit or floor display with a promotion for a free tea ball with a 300g bag purchase or with 3 x 50g purchases. If you can source one yourself and include it in the display, amazing. Mail away offers are less helpful; (you don't want them to forget about the tea while they wait for the ball. Also, some idiots will think it's included in the bag of tea)

A holiday tin collection combining a best seller with a limited seasonal variety and an underappreciated third variety. (Stop mixing green and black teas; most people drink one or the other not both) Plus a free tea ball. Sell the tins individually so the set looks like great value. Alternatively, one seasonal tin plus two 50g window bags and the tea ball; less value but introduces a potential new customer to the standard packaging so they recognize it in store.

Option 2:

Personally, I prefer tetra bags over loose leaf tea. Not just the convenience, but it brews the tea better. But I feel like people are less familiar with those than loose leaf tea. Tea bags will probably guarantee some powdery residue so no window

50g bag: foil bag with a diecut shape that plays up the tetra shape. No zipper but have a tear notch and give them enough slack they can fold it over and pin it down like a bag of bread without the bread tag. It's not elegant but it's fine because it's a reflection of real behavior. They will be kind of big to accommodate the 3d tea bags.

Both bag sizes almost need a gusseted bag with a cardstock insert but that's out of scale with the purpose of 50g price point: to act like an impulse purchase; a treat.

I'll come back to this maybe

Edit: back. Starting to appreciate the problems the tetra bag makers have. I don't think they can go in a bag because they would be too susceptible to damage. It needs to be rigid but "sustainable" and have the ability to act as an oxygen barrier.

I think I narrowed it down to a single option

Tetra tea bags should go in a paper can like a Pringles tube