Best Of

All three of them are good burners. I am not fond of Riellos myself but they are good burners. It's the parts they use I don't care for but that is just me. Most people love them.

As a point of reference, I maintain a 4-unit condo building in the Boston suburbs. We have two WGO-5 boilers that burn a total of about 1200 gals/yr, which happens to be almost exactly double your consumption rate. I've calculated our building heat loss 4 different ways, 3 of which used actual oil consumption or boiler run times at known outdoor temps, and all the heat loss numbers agree within 10%, so I have high confidence in them. Our heat loss is 90,000 BTU/hr, and since the math is basically linear, by comparison your heat loss (in a similar HDD zone) would be half, since you consume half the amount of oil. Which puts your heat loss at 45,000 BTU/hr, quite close to the 50,000 BTU/hr that @EBEBRATT-Ed predicted earlier.

We also have massively oversized boilers, and I plan on downsizing to WGO-2's when the time comes. But since you say you're doing the install and weight matters, you might want to look at the Biasi B10/3 or B10/4. Those are around 300 pounds, much smaller and easier to wrangle than the 800-pound monster UO-3. I do not have any experience with the Biasi, but I have done some reading on it in the course of looking for alternatives to our WGO-5's. There are a number of people here who have experience with Biasi's, and there are a number of threads discussing their reliability, etc.

https://qhtinc.com/wp-content/uploads/2022/03/B10-Lit-0321.pdf

The downside of the Biasi is that the plumbing is all different from your existing WGO setup, so if you can wrassle a new WGO in there like @EdTheHeaterMan said, that would be simpler plumbing-wise.

The Buderus distributors by me offer the Hydrostat 3250 Plus. If you're lucky, you can get one in Buderus Blue. You'll also need to change the well. Actually, make sure you specify what boiler you have because well insertion depth is critical. If the system already has an external LWCO, you don't need the Hydrostat Electrowell, but redundancy never hurt anyone.

A simple cheap blower door test WILL show where the cheapest and highest return on investments can be made.

After lots of measuring and calculating and filling out the form I got about 70,000 btus/hr. So now I've pinpointed my desired energy capacity to exactly precisely somewhere between 30,000 (fuel bill method) and 70,000 btus (this method)

yeah this is exactly why you should not calculate it by the assembly lol. Way too inaccurate to use for someone who’s doing it for the first time. Why not just trust what actually happened? Otherwise you’re relying on a bunch of guesswork.

Regardless, you know what to do now. Reach out to some contractors and good luck!