well dsm/xpenology is a appliance and in some points restricted, imho best alternative when it comes to plugins/docker (ZFS based is not a real comparsion here, dsm does not have this) is open media vault (omv), looks nice and it still my fallback if dsm is not going to work for me, it can use the lvm/mdadm raids created from synology so you can boot up ant instantly use your data raid (i tested this with a raid6 from dsm, not tested with SHR raids)
you can still buy a used 8port controller
lot of people use sas2008/sas2108/sas2208/sas2308 based controllers flashed to it-firmware (it in this case means Initiator Target) there a tons of cheap ones from dell (perc h200), ibm, fujitsu, hp (Hxxx series)
sas3004/sas3008/sas3108 will work too and all of them are very reliable
also working with 6.1, 8-port and not too expansive
Marvell 88SE648x based like the HighPoint RocketRAID 2680
Marvell 88SE948x based like the HighPoint RocketRAID 2720
Adaptec HBA 1000-8i, (PMC chip based?)
a save bet are ahci compatible controllers, often used are marvell based ones (cheap) but they usualy come with 4 ports, dont know if there is a "real" 4+4 port marvel based controller (two 4 ports chips together linked by a pci-e bridge chip) they are possible (i think) but i cant remember if i ever saw one, i guess the costs for such tandem will be bigger or equal as the bigger marvel chips like 88SE9485