Volume of x-boat traffic and fuel availability would certainly be major concerns. Without x-boat traffic information I'm not sure how you would calculate it. World population might determine volume of data, but not necessarily x-boats in the system. Does anything like that exist in Traveller canon?
The consensus developed over the last few decades pegs the system's operational tempo at
One boat per day per link. That tempo is determined more by the temporal accuracy of jump drive than any potential message traffic. X-boat messages are relatively pricey; the "Big Mac' rule puts the price of sending the canonical 20kb message one parsec at about 20 USD.
With stamps going for twenty bucks per parsec and other shipping heading to more destinations than the X-boats do, there isn't going to be that many private messages carried by the boats.
I imagine a world like Glisten, which is a hub for four x-boat routes, might have several stations like this or larger, or several tenders operating at once.
In this case, the physical accuracy of jump drive greatly influences the answer. Jump's physical accuracy is 3K km per parsec so, at most, a boat is going to arrive within a sphere with a 12K km radius. A tender with it's one gee maneuver drive can cover that volume in a relatively small period of time - especially when there is only one boat scheduled every 24 hours.
Glisten has four links; Bendor, Horosho, New Rome, and Overnale. With jump masking and shadows a canonical fact, each of those links is going to have a "best case" arrival/departure region within the Glisten system with respect to the four linked systems. As noted above, one tender will suffice to cover each region. (There will be spares for overhauls, breakdowns, and the like.)
I'm not sure if a fuel balloon is a solution either as it would eventually need to be refilled.
A balloon would need to be refilled, but it wouldn't need to be refilled as often as the canonical tender or your - superior IMHO - station. As it stands now, the canonical tender can fully refuel 3 boats before needing to be refueled itself.
It kind of just moves the goal post.
You can kick the can down the road or you can provide a couple thousand dTons of fuel storage in a balloon that only needs to refilled every quarter.
That's why I included a 50 ton fuel cutter as part of the station's vehicles, although I understand that it may be woefully ineffective depending on traffic.
As a chase boat or "runabout" the cutter is fine. It's fine even as a mini-tanker to refuel a boat the station doesn't want to dock with. As a refueling tanker for the station however, the cutter can carry 30(?) dTons of fuel while the station is pumping as much as 40 dTons as day. The cutter simply can't keep up.
I think the best scenario would be to have the station (or tender) orbit a gas giant - probably at 100 diameters so the x-boat could jump to it safely.
Think of the distance that 100D actually means, especially with respect to a gas giant. Saturn is roughly 110K km in diameter. Let's round that down to 100K and then apply the 100D limit. At the 4gees the cutter is good for, we're still looking at slightly over 27 hours for the trip one way. That's 27 hours to deliver 30dTons of fuel when you could be pumping as much as 40dTtons every 24 hours. It just doesn't work.
Precision navigation on the part of the x-boat pilots would minimize the amount of time spent retrieving their ships.
That doesn't matter. Jump's physical accuracy is 3K km per parsec so a boat's maximum deviation is only 12K km. A one gee tender can cover that in roughly 40 minutes.
Fuel shuttles would work as hard as necessary to keep the tanks filled.
They can work as hard as possible, there still isn't enough time.
Your design is better than the canonical tender on several levels. That being said, tenders and stations are going to have a big honking fuel "balloon" nearby. It's the cheapest way to ensure the necessary fuel is where it needs to be when it needs to be there.
