Is usury considered a greater sin, more dangerous, and more harmful to people than theft and embezzlement, and can theft and embezzlement sometimes be more dangerous than usury?

What you did by brokering the sale of the land is called samsara (brokerage), which is an intermediation between contracting parties for a fee. Brokerage is considered a form of ju'ala (commission/reward) according to jurists. For the broker's fee to be valid, it must be known. The majority of scholars do not permit it to be a percentage of the land's value, but some scholars have allowed it. If the contract is invalid, you are entitled to a fair wage (ajrat al-mithl). As for your statement that what you did was worse than theft, usury (riba) is more severely forbidden. Allah has warned those who consume it with war. This does not diminish the severity of theft; rather, it emphasizes that usury is among the gravest of major sins.