So after 3 countries, 3 car rentals, and a lot of potatoes, here's what I've learned regarding my original question:

In the last 17 years since that other guy's post, no rental car agency has figured out that a lot of us want to tour all three countries on an open-jaw ticket. Too bad.

All the major agencies allow you to cross the border into friendly states without a penalty. Rent in Vilnius, you can dip into Poland no problem. Want to drive from Valga to Valka and back? Won't take long and it won't cost extra. You can even go to HEL from TLL with your rental car if you feel like paying for that ferry and wasting a lot of time.

Yes, you may rent a car one way. It costs a ton more. No real point. Take the train from Tallinn to Tartu and between other cities. Where there is no train there is LuxExpress Bus. Free wifi, coffee and lav onboard. Prices are reasonable and you can easily work or plan onward travel from the comfort of your seat. Be aware that wifi cuts out for about half an hour around borders as you change 5G networks. Seems like each bus has a bonded 5G connection. Book at least 24-48 hours out to assure yourself of departure time and seat option. Book the lounger seat whenever possible, it's only a couple euro more.

I drive stick at home, and it seems as getting a stickshift in Europe is becoming more difficult. Only one of the three cars I rented had one, a silky sixspeed gearbox by Citröen.

You could fly from capitol to capitol if that works better for your itinerary. All three airports are relatively close-in. I wouldn't fly if your intention is to make your way through all three Baltic states. Just take the bus and train. Just as fast/Almost as fast, when considering luggage security, weather delays, et cetera.

Rent the car only for the days you'll need it exploring small towns, beaches, lakes, forests, and countryside. Tallinn, Tartu, Riga, Vilnius and Kaunas all have plenty of rental car agencies and you won't need or want the car in the city.

I rarely drove within cities because I seldom had a car rented for those portions of the trip. You could but be prepared for big annoyances regarding parking. Payment for parking only by app in Estonia as an example, but the apps don't work half the time. If you need a car within cities, just hail a taxi or order the local flavor of Uber: BOLT! The busses and pedestrian options are easy and faster than taking a ride-hailing service within cities. Everything is very close together.

Finally on my return to the Baltic, I may choose to rent a car and keep it my entire journey only if I'm driving through all three countries in a loop or figure 8. I'll leave the car parked outside of town at an airport garage if/when I'm in a major city. Rates are quite favorable and the garages are super safe.

Europcar desk at TLL airport was super nice, excellent service. Europcar desk in Tartu suburbs was pleasant and relaxed. Be aware that Europcar and a few others have rental car offices at Tartu airport for reason utterly unknown. Tartu airport is far from the city center and has no scheduled passenger flights. Skip it. Sixt in the center of Riga was very efficient and friendly but logistics were VERY inconvenient, just go to the airport location. Europcar in VNO airport were really annoying. Told me only on arrival at the desk I couldn't use AMEX for the deposit, even though the website had accepted my payment with AMEX. So the deposit was held on my Visa, which was frustrating because I knew that if anything went wrong with the car, there could be an issue over which card's auto rental insurance scheme would cover me. Whatever, no dents, so fortunately no problems.

Lithuanian countryside driving, which could be fun, is littered with average speed cams. What a headache.

Estonian drivers were very polite. Latvians a slightly more aggressive. Lithuanian drivers on the other hand… look out. My guess is they are all pissed off about the speed traps.

Hope this is helpful to others. Travel safely and remember: eat all the potatoes when you're in the Baltic.