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It doesn't take a lot of tank to hold hydronic pressure and temperature. An expansion tank for example is sheetmetal.

I built a solar drainback tank from a Harbor Freight air tank.

Heck a .11 mm aluminum Coke can, about the thickness of a human hair, can hold 90 psi!

For longevity I would lean toward glass line . Stainless tank makes sense . Welding stainless is difficult which , Which brings up the percentage of an issue …

Make the size increase short and close to the tank. You get a slight velocity increase through the increaser.

I've taken 3/4 to 1-1/4" on tanks 12- 14 gpm possible.

Here are the fittings I have used on HW tanks.

3/4 mip to 1-1/4 copper adapter use on the water connections

3/4 x 1-1/2 stainless bell

1" mip x 1-1/4 copper use for the element holes

1X 1-1/4 brass swage nipple

Swage nipples were used in Munchkins as their header was 1" connection. So some were 1X 1-1/4 and some actually were 1 X 1-1/2" nipples! I think the 199,000 had the 1X 1-1/2 swage nipples

On this 6 gallon tank, the 3/4 water connections are increased to 1-1/4 for the LLH function

Stainless tanks? How are your chloride levels?

Glass lined seem to be able to handle most any water condition. Keep them under 180°F. High temperature, thermal stress, is a killer on glass DHW tanks.

Is the boiler connected with CSST gas pipe?

Thanks @mattmia2 and @EricPeterson

I agree that it seems to perform great for that purpose the temp at the boiler gets up pretty quickly and as it gradually opens to full flow it avoids any shocks due to the big temp difference coming down from the radiators. I agree that it seems like a small investment in an insurance policy that protects the most important piece of the system.

Thanks Again