Fix for this has been pushed and is live. If you refresh, you'll see it (although I'd recommend exporting / copying your save so Itch doesn't wipe it).... you are now no longer gated by number of deliveries to tier up, you can also now adjust the 'max tier' incase you buy too many, and 0 weight donuts now actually reward (before they were at $0 and just filler).
I entered the donut hole and started a new run. Deliveries feel somewhat better now, but I think there could still be some improvements. Right now tier 5 costs 2.5e9 when, currently, I can barely fulfill tier 4 and tier 4 give e9 payouts, which makes it feel unbalanced. There are a few routes you could take to improve this and some combination of them would probably be best.
One, you could increase tier up costs; they should probably cost at least one more magnitude to make it so delivery requirements aren't too high for what's possible to generate. Two, you could increase base donut stockpiling rate or reduce base delivery requirements, which accomplish essentially the same thing; if deliveries are easier, there's more incentive to use higher tiers since they feel impossible otherwise. Three, you could decrease costs for new donuts and weight increases to push donuts to higher production quicker and match those costs to needed generation and current tier payouts better. Four, you could decrease delivery payouts to prevent being able to afford a tier with unreasonable delivery requirements for the current point of progression.
With the right combination of those four, delivery payouts wouldn't out pace the max tier you can afford, and delivery requirements for the current max tier wouldn't out pace the amount of donuts that are reasonably possible to generate with the amount of money you can generate. Delivery requirements should still out pace it somewhat though, to give space for increasing capabilities with further upgrades at the start of a new tier and not make progression too quick.
Delivery payout scaling also needs to be able to keep pace properly within a given tier. If scaling kicks in too early and/or too aggressively, it would make affording tier ups and the necessary upgrades to keep pace in the next tier too slow. On the other hand, if scaling kicks in too late and/or not aggressively enough, the reverse is true; upgrades and tiers would come too quick. Currently, I think it kicks in too aggressively because payouts drop below $PS when underdog logistics kicks in while upgrades cost two or three magnitudes above $PS. After some of the above changes, it might be ok to leave as is, but if payouts get decreased, scaling definitely needs tuning.
If you balance it it right, shop progression would feel smoother overall and match how smooth progression feels before unlocking shop. Before shop, nothing felt like it needed idling, but at points in shop, it has.
That type of feedback is invaluable, and I've spent the last 3 hours tweaking values and play testing the whole phase, and I think I've come up with something that makes way more sense and plays much better with way less downtime. There is still a smidge of 'idle' time, but maybe a combined 10 minutes in what should be about a 90 minute run to get through the shop, and that's just waiting for massive delivery orders to finish to get huge money. basically remapped A LOT of costs across the board, increased delivery payouts in the middle tiers, decreased some shop costs and increased others. just played through the whole thing start to finish and it went quite well. new build is uploaded to Itch so perhaps if you make it to a 3rd play through, it should feel better (and obviously way faster with the buffs lol)