Or choose a Turnstile type antenna but I will have to choose between a 144Mhz and 430Mhz antenna.
I have the impression that there are more satellites in the 430 Mhz band but are they easily accessible for this type of antenna (non-directional and without tracking)?
Therefore:
I keep my current antenna?
I replace by a DIAMOND X30 & X30N?
I replace by a turnstile in 430 Mhz to have a significant improvement and to access more satellites (having positive results) for this band (and I abandon for the moment the band of 144 Mhz)?
Going from a discone to a Diamond X-30 will not improve matters that much - both antennas have patterns narrow to the horizon, so you end up with a large portion of the pass where the signal will be weaker.
Moving to a Turnstile is a better option - though you will need to use a preamplifier.
As vk5qi said, you need an antenna with high angle of radiation.
I found my discone was a poor performer for satellites.
I tried many others and ended up with a pair of home-brew QFH antennas which get good omni directional results.
See my ground station 568 and 724.
Yes the 2m band is overcrowded and 70cm is becoming very popular.
I’m surprised how well my UHF QFH is working considering it’s such a small antenna made from some wire recovered from an old transformer. 3mm copper wire I think.
I do have LNA4ALL pre-amps at the antennas and I’m using SDRs and RPis to do the decoding.
73 Bob vk2byf