Oh yes! They are overpriced even on the secondary market. And they of course don’t make ram, I think ADATA used to be their vendor, now someone else; they literally buy ram from the next door maker who has it cheapest that day, slap their sticker and suddenly it’s worth its weight in gold.
When I used Synology I would immediately remove ram from it, sell it on eBay and proceeds would cover 4x size of non-Synology memory, sometimes even branded. That was surreal.
That said, early models (pre-DDR4) had some peculiarly configured controller requiring single rank sticks. I don’t think there are any restrictions with modern ddr4 models, but I would go with sticks confirmed as working here Synology NAS Memory Upgrade Guide – 2020/2021 Edition – NAS Compares or at least with the same specs (frequency, latencies, rank) — it’s now more important since you can’t remove soldered 2GB portion.