Can I refill cartridges instead of buying OEM ones?

Refilling cartridges absolutely works and tons of people do it successfully. The warranty thing is a bit of a scare tactic on Brother's part - they can't actually void your warranty just for using third-party ink, at least not in most jurisdictions. That said, if something goes wrong and you're trying to claim warranty service, they might give you grief about it. The real risks are pretty minimal though: worst case scenario is your prints look slightly off in color or quality, or a refill leaks inside the printer (rare but possible). I've known people refilling for years with no issues.

The key is just getting decent quality refills, not the super cheap ones. Amazon has plenty of options and the reviews will tell you which brands actually work well with Brother printers - look for ones with good feedback from people using your specific model. When you refill, make sure you're following the instructions carefully: clean the nozzles properly if the kit includes instructions for that, don't overfill the cartridge, and let any adhesive or seals set if required. Some people just swap out cartridges and never have a problem; others take a bit more care with the process depending on the kit.

Money-wise you're looking at maybe 80-90% savings if you go the refill route, so even if you mess up a cartridge or two while learning, you're still way ahead. Just keep some paper towels handy in case of minor spills and don't stress too much about it. Plenty of people run Brother printers for years on nothing but refills.