You are right, Instance options - Incus documentation describes it very clearly.
running incus config edit VMNAME and adding the following
config:
(...)
raw.qemu.conf: |-
[memory]
maxmem = "65536M"
(...)
solves the problem 
FreeBSD VMs now booting up perfectly fine with > 2 GB RAM on Incus 6.14.
Thank you for your fast support!