Congratulations PBA, on your purchase.
The typical domestic pizza stone has about 6% the specific heat factor of a thick baking steel. However, do you need one is a more complex question as it depends on your style of baking and the formulae you use. Some cooks find steels can burn crusts at high temperatures leaving a bitter flavour.
Check the dimensions of your new oven, particularly rack size to ascertain what size stone/steel will fit. I suspect you may have trouble finding something small enough to fit the racks. Someday Anova may consider them as useful accessories. You may have to contact Anova as they don’t disclose the precise details you need to know. I would ask for their recommendation, - and why, particularly regarding your steam related issues.
While you’re in contact with the Anova folks you also might want to ask about their recommendation regarding water filtration. Nothing kills a boiler faster than a heavy accumulation of sediment and salts. It may be important to keep the warranty valid unless they have some way for you to purge the boiler on a regular basis. Don’t even think about using any caustic compounds for boiler maintenance.
FYI, commercial combi-oven manufacturers (Alto-Shaam and Rational) provide 1-day and 2-day educational workshops for profession chefs so they have sufficient knowledge to use their ovens successfully. Is something similar available on the internet?
Happy cooking, - and keep well.