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Given the application, would perhaps a possibly too high delta T cause an inordinate amount of moisture on the combustion side of the vessel ? This causing the traditional issue of the by-products of combustionn ie ; sulphur, soot (unburned fuel), and ash adhering to that moisture ? In a extreme condition the weight of such impurities might have ill-effected the positioning of said burner tubes ? As with the preferred modern piping arrangements on the new smaller heat exchangers, maybe a primary, secondary loop piping set-up would alleviate the condition. And excuse this old timer but, isn't that one Grundfos circ pump installed in an incorrect position ? I'll believe so.

Yes, infiltration is a guess without some type measurement like a blower door score. Both EdTheHeaterMan and bio_guy have good points. Ideally, you could use the heat load calculations and the fuel use calculations to produce the most educated guess that is closest to the true heat load.

boilers work best when they have no leaks and never get the fluid changed unless there is something wrong with that fluid such as old or cooked glycol. You do however want to service the burner, typically annually or every other year. I like to do a burner service after the first heating season personally to make sure everything seems normal. Part of that service in my opinion should be cleaning the burner, flushing the condensate trap, checking the neutralizer if installed, checking the static fill pressure to make sure it is still in range (no leaks) visual inspection of the cabinet, visual inspection of the vent system, visual inspection of the piping system dynamic/static gas pressure checks and a combustion test making adjustments if needed. It looks like a lot of stuff but really cleaning the burner is what will take time, the rest is just making sure everything is still normal or in range, maybe an extra 45 minutes or so tops unless something is off.

What Edthehearterman wrote about lots of radiation. Fortunately, lots of people are in a good position that way with increasing their insulation and air sealting their homes

I have a problem with some of the other advice. I don't see How can a load calculation can come closer to boiler capacity needed than a determination based on fuel use, boiler efficiency and heating degree days. There is, of course, the problem of calculating in the DHW and cooking contribution if the same fuel is used for those things. With an old house, expecially without an energy rating for the structure, infiltration is a guess. Insulation quality may also be a guess

Can’t go wrong with a Fluidmaster. They beat out the brass ones by a mile. Easy to install and service.