Up to you, but it’s quite straightforward to do yourself. As a Belgian citizen you don’t need to open a bank account and prove any funds in a Thai bank if you can meet the required income 65K baht a month and get a affidavit letter from the Belgian embassy in Bangkok. Get a hand out of the requirements from the immigration before you apply. If you insist on using an agent as I understand the owners of this site Thai visa Centre also assist klients in Chiang mai.
Print out your latest tax return or annual statement from your pension provider and send it or email it to your embassy for a affidavit letter if your embassy offers this service.
As said, it depends on the IO you meet and your previous travel history. Even if an embassy/consulate issues you a tourist visa without any insight in your previous potential excessive travel history, the immigration will not be obligated to let you enter on this visa. Remember holding a visa to thailand in your passport doesn't mean you get in, all it means is IF they let you in, you get stamped in for the days the visa allows you.
These are the general requirements, when you fill in passport nation and current location, the e-visa system directs you to the embassy/consulate that handles our area. Contact them and ask. Where are you by the way?
Nothing is guaranteed even with a tourist visa, as everything is in the discretion of the IO you’re standing in front of and depends on your previous travel history.
I think you will be fine with a multiple entries tourist visa now. Just be prepared to show onward and return tickets upon entry if they’ll ask you.
-Professional landborder company, yes that’s a border run with a company that prepares everything for you and make sure you’ll be able to return with them on a new 60 days visa exemption. (Price 3500 -5000 baht)
-Safe airport entry or prescan is a service at the airport where an agent prepare your entry and escort you through the immigration and make sure you’re allowed to enter on a new visa exemption. (Cost 7-8 000).
As said if you don’t have any excessive travel history beyond the 2,5 months my guess is that you’ll be fine with a multiple entries tourist visa for your planned stay now and might just risk to be told to get a proper long stay visa as a DTV if you plan or wish to do further long stay in Thailand.
You need your original birth certificate from Lithuania. No impediment to marriage is a certificate proving you’re singel and not still married. Capacity to contract marriage is a certificate proving you fulfill formal requirements for marriage, normally just that you’ve passed 18 years who is the normal age limit. Extract from the register of population is just a formal letter proving you’re a Lithuanian citizen from the population register.