Payment of Goods sold on credit.

Since there is nothing in writing, it is very diffcult to prove your case. However, you can take a chance by sending a notice immediately with details and if not rebutted, you may have some chance to pursue your case. 

For future, you need to decided the terms and conditions of your future contract upon discussion and consultation with a lawyer, preferably. It is not advisable to use copy paste generic contracts as you may miss certain points suitable exclusively for your case. But remember, having a generic contract is better than having no contract at all.

You may contact a lawyer with all facts and documents (whatsapp msg/ bills etc.) and get a notice drafted by him. 

You have a  good case and everything is in writing as the wtsapp messages are also part of the evidence hence it shall be better to engage a local lawyer dealing in civil matters and got served a legal notice to the defaulting party and demand the dues with interest and cost within a stipulated time failing which file a civil suit before the competent court of law.

In future, clearly make a contract with every parties containing clearly terms and conditions and consequences in case of default by any of the parties. 

The I-T Act recognises electronic communication as proof in court. ... Legal notice or messages sent through WhatsApp messaging app are valid legal evidence under law, and the blue tick over the messaging app is a valid proof that the respondent has accepted the physical copies of the communication, said Bombay High Court

It all depends on mutual relations between you and the restaurants and previous experience.  I believe that these difficulties were only during 2020, as lockdown and COVID 19 menace have started, and Pune was reeling under stress due to lockdown of many institutions including hotels.  It appears that as of now you do not have any problems and the problem is only recently attributed to COVID 19.  In business, there must be understanding and the dependable customers are very few in your business and maintaining a relationship is the key to the success of your business.  It all depends on your selection of such clients and if the client is bad, any number of precautions to take legal action becomes futile.

In trade, such transactions are common and it is seldom that agreements are dishonoured. The non-payments could be due to adverse circumstances. Anyhow, you can serve the defaulters notice through an advocate bringing all the facts on record.