While I have been stalling for days, I have made some progress on the issue tonight.
It seems the problem is related to the UDP sessions timeouts. Because I have some VOIP phones and their sessions were expiring, I had to set the firewall optimization options to "conservative", thus my UDP states were taking someting between 300 to 900 seconds to expire. And L2TP/Ipsec is UDP traffic as well, making me beleive that was a concurrent session problem.
Now that I have set the firewall optimization options back to "normal", and adjusted the specific timeout of udp states to a much shorter delay than "conservative", but longer delay than "normal", I am able to connect l2tp sessions much more frequently and sometimes concurrently. The wait penalty is still painfull though. And my phones seem to remain online so far.
I know the best option would be to shorten the SIP phones polling interval and let the UDP state delay to normal, but my VOIP provider has locked this control on the phones, so it is complicate.
An ideal solution would be to be able to tune the following properties inside firewall rules if there is a match : UDP First, UDP Single, UDP Multiple.
This way, it would be possible to increase the UDP state timeout only for the VOIP traffic, but I don't know if it is doable at low level.
There exists a state timeout setting in the advanced firewall rules GUI, but unfortunately it is for TCP only.