Here are a number of things which we did not know before and knowing it would've been very helpful.
Belgium is a very slow country when it comes to processing Resident Permit. Especially if you're going to live in a city like Antwerp or Brussels it takes a lot of time.
We applied for and submitted our documents on November. After a month we got a date of appointment to visit the commune in February. 3 months just for appointment. The process starts only after that and it might go on for another 3 months easily.
The salary of 3k gross per month which most people ask here in the forum is just par for a family. Rents are exorbitant and it is also little difficult to find homes that people will rent for non-dutch speaking people with only their home passport. So you will not find homes and the best price. There's also home insurance which you've to take. Add 100 euros minimum extra to your calculation.
The lower wages in Belgium are compensated with a company car which you will get as cash in other countries. You can't get the car if your driving license is not valid in Belgium. Indian driving license or IDP doesn't work here. And you can't take the driving license test without your eID or national register number. Even the register number on your work permit won't help. There's a queue for taking the test in English. Most people don't crack the practical in the first attempt. There's also a collusion between driving schools and evaluators just like in our countries and you will end up paying more. The book in English costs twice the one in Dutch. So everything together 1000+ euros a bare minimum and up to 2500 euros. Or you don't get that benefit.
Speaking of the language, everything will be in Dutch or French. From foods to phone calls, sign boards everything. Even if you have to take a hospital appointment be prepared to hear things in one of these languages. I have lived only in the Flemish side and can't speak of the other. Honestly, there is a little discrimination based on language. Not from everyone, there are a lot of nice people too. But there is.
Coming to doctor, there is insurance for normal clinic visits too. Visiting a GP without insurance costs 26 euros. Since dependants don't have register number in any form all clinic visits and medicines will be out of pocket till they get resident permit. People coming with young kids beware. New environment, new microbes means a lot of doctor visits. And a doctor visit can cross 100 euros for a pediatrician with a blood test and medicines. Also sick leaves for people who work are paid from taxes/social security. So you need a doctor's note even for a single day sick leave. So your sick leaves have some cost above the insurance amount. Some companies need doctors note for 3 days leave only, so check with your company before coming.
Speaking of holidays, Belgium has one of the lowest holidays in EU. 20 days. But that's more than enough. The only problem is you don't get it in your first year. Yes, there are no leaves in the first year of work in Belgium. In Belgium you work a year to get your 20 days holidays. So the first year it's nil. This is so much pain. The EU scorned at this and made Belgium give something like European holidays to compensate but unfortunately it is unpaid. Even if money is not your concern, you may not be able to take it because as a highly skilled worker you need to be paid a sum which is, guess what, 3k per month. So unpaid holidays bye bye. Whatever happens, you shift house, your kid falls sick, someone in your home country dies, you are left to rue without any leave.
Everything here is based on appointments. So it's like you wait for appointment and the you wait for the work to be done. So have plans already to open bank account. Also it's better to land straight in Belgium by flight and not Amsterdam or Paris and take train/bus. This helps you get Sim card straight in the airport. Not all places have Sim card. A lot of shops with mobile phone ads sell top ups/recharges and not Sim. When you are traveling with a kid bring a stroller and travel lighter on luggage.
Be prepared for 3 months rental advance too as many house owners are asking for 3 from this year. Without RP day care, vaccination for kids is also a problem.
Belgium is also a largely car driving country. Not sure why as they still buy cars mostly manufactured elsewhere. So public transportation is a pain other than in top cities. Places that take 15 mins in car take 1+ hours in bus and it's almost never on time. But it's well connected and if you can wait you don't have to walk much.
A lot of these problems are likely to be solved with the single permit. But guess the single permit might itself be a pain as I can only imagine how much time it will take. The commune advised applying 4-5 months early for renewal. Some of these issues don't apply to people coming onsite.
I might have also missed out a number of things. So others feel free to add. And if I have got it wrong do share better ways. Everyone reading can benefit.