Happy new year.
Conjunction: Mars / Hades
isangle(mars, hades, 0, 3) > -1
3 degrees orb
Opposition: Jupiter / Kronos
isangle(jupiter, kronos, 180, 3) > -1
Midpoint: Jupiter=Saturn/Admetos
midpoint(saturn,admetos) > sun - 1 and midpoint(saturn,admetos) < sun + 1
Sum: Sun=Mars+Venus
mars+venus > sun - 1 && mars+venus < sun + 1
the previous 2 won't work for conjunctions around 360 degrees.
Other examples involving more than 3 objects.
isangle(mars, hades, 0, 3) > -1 or isangle(jupiter, kronos, 180, 3) > -1
For aspects, have a look at the new Time - Extended ephemeris, it can do aspects.