Your post sounds scrupulous. I can tell you are melancholic and your personality is inclined towards scruples. I hate to joke about a thing like this, but I'm feeling very sorry for your confessor(s). Seriously, they have their work cut out for them.

Buy gas whenever you need it. God doesn't expect us to be stranded on the highway -- not even on Sunday.

You are not obligated to travel 3 hours to Mass. However, if you're willing and able to travel that far for Mass, you shouldn't let a little thing like refueling stop you.

You are not causing someone to work by stopping to fill gas. You aren't even using the services of a cashier.

Full-service gas stations are a thing of the past. Most fuel transactions are done with 0 human intervention. Gas is pumped by machine using electricity.
Payment is usually done by credit/debit card, etc. Unless you're an illegal alien or something who lives outside the banking system.

If you're feeling scrupulous, just forego buying overpriced candy, 2-day-old hot dogs, soda, and other junk food "inside" where the 1 cashier is working.
That way you're not even *contributing* to the demand. The gas pumps could operate with no human on-duty.

What about going out to eat on Sunday?

I've seen both Fr. Gruner (Fatima Center) and a Sedeprivationist priest (Fr. Christopher Spray) say this is acceptable, and I've also asked my own Byzantine Priest and he said this was acceptable, so I assumed this was just acceptable.  I could be missing something though.

(I am not the OP)

With regard to eating, here's the way I look at it.  Eating is necessary.  So doing work in order to eat is acceptable.  But if you choose to go out, you're just transferring the work from one person to another.  Money isn't the key concern.  If you're transferring work, then it's acceptable to compensate the person for the work you offloaded from yourself.

Gas may or may not be necessary, but it's acceptable for gas stations to remain open on Sundays in case people do have the necessity to buy gas.  Since it's already OK for them to be open, then it's creating no additional work for anyone to go in there and fuel up ... especially when you do the pay-at-the-pump option.  There's already an attendant there minding the store, so you're not causing more work.

Are you a material participant in someone else's violation of the Sunday?  Perhaps.  But perhaps someone has a legitimate necessity to work on Sunday, e.g. needs an extra job to make ends meet for the family.  Or perhaps an attendant isn't even a Christian but some infidel (as may gas station attendants tend to be).  In any case, I am not required to police everyone who works on Sunday, go into the store and interrogate the attendant whether or not he has sufficient reason to be working there.

Shopping on Sunday is becoming an anachronism.

My future grandkids will ask "What is Shopping?"

Seriously, we order *everything* on Amazon.com or another online source if possible. There are only a few categories of items we can't buy online:
Things heavy/bulky (difficult to ship)
Things cheap (hence it's difficult to economically ship, even for Amazon -- adding $4 to a $1 or $2 item is hard to mask)
Food, perishables
Gas

But all the stuff my parents used to buy at K-mart, and we used to buy at a department/general store like Wal-mart -- all those things are Amazon now.
Pans, everything for the kitchen, Christmas and birthday gifts, books, parts for my outdoor equipment, electronics, computer parts, and hundreds of other things, too many to list. My first option for anything is "can I buy it from Amazon". The price is lowest, convenience is at a maximum, and I have the intelligence (as in, "military intelligence") to make an informed decision that I never had 20 years ago. Customer reviews are a priceless resource. I don't know how we ever lived without them! Actually I do know -- we used to have to learn about product faults and limitations "the hard way".

It's even more important for those of us who live in a rural area.

I also love having a record of everything I've ever purchased. I can do a search through my past orders and see what model/brand I bought 5 years ago, and buy it again.

P.S. We do this much online shopping, but for my wife and I PRICE is the main priority. If we were willing to pay a bit more for various things (read: more than the absolute lowest price) we would buy even MORE stuff online. Groceries, for example. But, frankly, Amazon has the lowest price on most goods AND they have the most convenience. I don't have to waste an hour driving to/from town, and I get the item delivered the next day. Hard to beat.

I've seen both Fr. Gruner (Fatima Center) and a Sedeprivationist priest (Fr. Christopher Spray) say this is acceptable, and I've also asked my own Byzantine Priest and he said this was acceptable, so I assumed this was just acceptable.  I could be missing something though.

(I am not the OP)

God rest Fr. Gruner's soul.
God bless Fr. Spray.