A cavity duplexer is, basically, a set of very sharp notch filters. The stereotypical application for a cavity duplexer is as part of a repeater system. Repeaters tend to have a few characteristics which make cavity duplexers ideal:
1) Full-duplex operation
2) No frequency changes
3) Receive and transmit frequencies are significantly different
4) Usually a shared RX/TX antenna, for cost, tower congestion, and link symmetry reasons
5) Hostile RF environment, with other nearby spectrum users on the same tower
It's important to remember that a cavity duplexer is a notch filter, and it's not a very agile piece of hardware. It's manually tuned for a specific frequency pair, and changing this requires, at best, hand tools and a bit of elbow grease. When you're only intending to use a single frequency pair, there are MUCH better options out there than a bladeRF (or any other frequency-agile SDR). So I'll assume a cavity duplexer is not for you.
In your case, using separate RX and TX antennas is entirely doable, especially if the antennas are in each other's nulls (e.g. vertical antennas separated vertically) and they're operating far enough apart frequency-wise. Obviously this means you've got twice the feedline and twice the antennas to deal with, but it's full-duplex and it may very well be good enough. Filters are, as always, a good investment, but they don't need to be anywhere near as sharp as a cavity.
If you don't need to work full-duplex and can get away with muting RX while transmitting, triggering a transmit/receive switch via GPIO is a splendid option. This can be quite useful if your amplifier has a LNA pre-amplifier you'd like to use, too.
Or, if you're sticking within a particular range of frequencies, a circulator might do the trick for you. It's another kind of duplexer. Depending on how much amplification you're planning on doing, though, one might not provide adequate isolation...