If you’ve just got your
218+ and you’re not outgrowing it already (not only storage wise but capabilities wise too), I’d suggest that you wait.
Use it and understand the various configurations, features and capabilities (and limitations). You’ll be in a better position to make your second purchase more useful for your (discovered) needs. Or you may find that what you have is good enough and there’s no need to buy anything now.
SynoForum is a good place to learn. Check the resources section, use the search function and of course you can ask anytime.
However, if you must buy now, go for
920+ for a four bay (not
918+) or a
1019+ for a five bay.
The last two digits in Synology’s model numbers denote the year of introduction. The first digit(s) are the maximum number of bays supported (sometimes with an expansion unit).
A 1019+ was introduced in 2019, can support a maximum of 10 bays (with the optional 5 bays unit connected) and it’s a plus series.