That is false, the rulers were the ones that accepted Christianity,

If you acctually study history, you would see storyline was bit different. First one part of society accepted. Soldiers who fought in foreign service, merchants who traded with foreigners etc etc. They would construce basis. Latter, we had missions, offten multiple. In end, ruler would accept Christianity for political reasons or out of sincere conviction, or sometimes forced by foreign powers. He was supported by one side of society and other was resisting. But clashs, and sometimes wars were of no epic proportions.

blinded with the wealth they were offered,

What gold? Why would some state lets say Byzantine Empire, in dire need of money in order to support its army constantly involved in warfare, offer gold to some other contry which was more offten on enemy side, than allies?
One of reasons for Evangelisations, was establishing of peace, and to avoid tributes which Slavic princes were extracting from neigbiourging arreas.

or they simply wanted to get married to a Christian princess, but the princess naturally wouldn't want to convert to Paganism.

Yes and they wanted to establis alliances with foreign powers and then improve their rank in Europe. But, of course foreign powers were interested to establish religious and cultural links as well, or more precise to deapen it.

The rulers and their heirs persecuted those who followed the national religion, but found it difficult.

Well, first, I hope you realise it is time when peoples were forming. Nations got formed some 1000 years latter, so from purely theoretical stance, you are wrong. Now lets take factual side. Yes there was persecution, but it was two side, many missionaries were killed, by their opponents. And it was not like there was some long lasting conflict. Conversion of most Slavic tribes was peacefull, by standards of Middle Ages of course.

In Serbia, the last Pagan temple was demolished during the reign of Tsar Dushan, in the 14th century!

I heard/red that sentence thousands of times, but never heard for source of such claims. I mean, there must be some chronicle, charter, whatever. But there is nothing. Next, time please, try to provide source.

Fortunately enough, our faith is recovering, although quite damaged.

I dont really wish to go in it. From my experience it is just new age with Slavic flavour. (sometimes with lot of Dharmic influences)

Romans were polytheists, not pagans.

Polythheism is whorship of multiple gods. In IV, V, VI century, Christians from Rome were calling their immediate neigbourghs, who lived in rural arreas of Latio, and who followed Roman and Latin Polytheism pagans. So from that perspective, and from perspective of English language, I think wrds are synonym.

One can say that Slavic paganism is also polytheistic, but in it's essence it's not.

In esence it is polytheistic. Also in practice and theory.

Our paganism is more pragmatic, tribal and naturalistic (spiritual), wheres the Roman polytheism was in written form, and was meant to spread moral values (similar to Christianity).

You cant find more pragmatic religion than Ancient Roman. Take for example word religion, religio in Latin. That was contract between gods and Roman people, in exchange for sacrifices and offerings, Roman gods were offering protection for Roman people. Also, religion was public duty, not matter of conviction.

Per, tirbalistic and naturalistic it has lot to do with cultural development. Romans lived in cities mostly, so its bit hard ot preserve naturalistic and tribalistic characther. Which was still verry tracable. Up until empire, many importnant priest offices were confined just to certain clans (curias).

And,  for moral values, well, Roman religion did not have ethic teaching. Of course, it was respecting of customs of fathers, mos maiorum. But again unwritten code of of honour is bit a tribalistic thing. There were of course Roman morlaists but they were philosophers, and not offshot of Roman religion.

Apropo, wirtten form, well, Roman Religion did not have Holy Scriptures in Jewish fashion. There was plentitude of myths about deities, their origin and deeds, but they were contradictory, and never held to had canonic authorty.