Buying and selling in a short amount of time is often a bad idea with modular. It is better to keep an unused module in the rack or in storage than sell it and buy something else. A modular system is greater than the sum of its part, and a disappointing module might reveal some usefulness once other modules come into play. It will also help you to keep a budget and avoid endless GAS. Of course once your system stabilise selling unused things is fine :)
Sequencers can be thought like modulators: don't use them for notes only - anything that can be cv'ed can be sequenced, which can bring extra layers of expression to your music.
First thing worth considering is sequencing from external devices. Often cheaper, they can also be helpful in bridging the gap between the modular system and the rest of your setup.
This includes midi to cv modules (for example: 2hp midi), and hardware like the Arturia Beatstep & Keystep sequencers.
Sequencers often need a trigger or gate to work, being able to do more than a simple regular clock can bring more variation to your patterns.
Useful to make repetitive patterns of notes, they are very easy to program and can feel quite limited, but there's ways to explore them creatively.
The basic idea of generative sequencers is the sample and hold circuit. In its typical use it will generate a sequence of random values. Feed it an LFO and a synced clocked signal and it will become a repeated sequence.
There's a few notable variations to this that are interesting to look at: