I took one from Corsair, MP540 with 240GB. I would pay particular attention to the max TBW values (Total Bytes Written) and take an SSD with the highest possible value. If these TBW are used up, Synology refuses further use and you have to buy a new SSD! The Samsung ones don't really shine (240GB Samsung 970 EVO plus with 150TBW vs. 240GB Corsair MP540 with 400TBW). I would also check the SSD Cache Advisor (or similar) beforehand to see which SSD size is recommended there. This can be found in DSM > Storage Manager > SSD Cache > SSD Cache Advisor.

I ordered the "Samsung 970 EVO (500GB, M.2 2280)" and it works great. How big the ssd has to be depends... If you already have a synology and buy it as a replacement, then you can charge for it.

Many thanks