hello Matteo - welcome to the GMB, and compliments on your English
............. now Milan I've heard of, but regret that Forte dei Marmi - despite your comments that it's famous - has escaped my education, until this week

You may well be correct with your suggestion that this was from the Gulf of Venice, though regret not my area, and as am sure you will know, the prodigious quantity and variety of glass that has come from the 'Island of Murano' is truly staggering, though bearing in mind your location then a 'Murano' provenance is possible .............. if my understanding is correct, then the word simply implies one of many glasshouses on the island, and most glass from that location that reaches the books and auctions appears to be the more colourful art glass, and of course all the animals.
With unmarked glass, and an absence of labels, we are into immediate difficulties as to attribution, and id for your platter/plate will require that someone somewhere recognizes it and is able to confirm your suggestion. Assume you mean 1.5 kg. of weight rather than 15, and
was it the dealer who originally suggested Murano and a date of 20s/30s, or is that speculation on your part?
The two art periods you mention are recognizably different, and I wouldn't have thought your glass old enough to be nouveau, but deco is a possibililty. Large plates/platters do occur in the 1930 - 45 period - Whitefriars and Nazeing - though British - did produce items that were similar in general shape during that period, some with substantial diameters, so your piece might be from that date, though such pieces have been made since.
Unfortunately, antiques dealers are not known for their accuracy when dating vintage items, so my feeling is that you must be cautious with a date. Fingers crossed someone may be able to offer real help.