Can I swap my AGM battery for a normal one to save some money?

My 2019 BMW 3 Series has been telling me for a week that the battery is getting weak. The dash shows a small warning when I start the car, and twice the stop start system stopped working on its own. I called two shops to ask about prices, and the difference confused me. One quoted a normal battery for a fair price. The other said the car needs an AGM one, and that it would cost almost twice as much. The first shop told me AGM is just a sales trick and a normal battery will work fine. The second said I would be back in six months if I did that. A neighbour who owns the same car told me to read about top Varta AGM battery technology explained for Dubai drivers before I decide, because he said it helped him understand why the price is higher. I read a page about AGM batteries online that evening, just to learn the basics. It said they handle heavy electrical use and constant stopping and starting better, and that they cope well with heat. It was easy to follow, but it did not say what actually happens if you put the wrong type in a car like mine. So I want to ask people who have been through this. Can a normal battery really cause problems in a car that came with an AGM, or is that just talk from the shops? Will the stop start system stop working, or will the car give warnings all the time? I also heard the car computer needs to be told when a new battery is fitted. Is that true, and does it cost extra? How long does an AGM last in the summer heat here compared to a normal one? If it lasts twice as long, then maybe the higher price is fair. Another thing is the date on the battery. How do I check how old it is before I pay, so I do not get one that sat on a shelf for a year? Do some shops sell old stock as new? I park outside at work, and most of my trips are short, around fifteen minutes each way, so I wonder if that is hard on the battery. The car has about 85,000 kilometers, and the battery is the original one, so it is almost five years old. Should I also ask them to test the charging part of the car, in case the real problem is not the battery? I do not want to spend money and then find out something else was wrong. If you changed your battery recently, please tell me what you chose, what you paid, and if you are happy with it now. If you tried the cheaper way first, I would like to hear how that went too. Even a small tip will help me a lot. Thanks for reading, and sorry for the long message.