mini-itx brings some limitations when it comes to extensions as it does not have much pcie slots (usually just one)
you did read about the general limitations of xpenology?
https://xpenology.com/forum/topic/13333-tutorialreference-6x-loaders-and-platforms/
when doing docker and vm then ram is more important, very few people use ecc as most build low cost systems with desktop hardware that does not support ecc, can't hurt to have it when its more important
is it worth the money? depends on how much more it costs and how you see it
here we have the 1st problem, if you look for the limits you will see only 918+ supports nvme (and only as cache and manually patching) but its kernel is limited to 8 threads
even 3617 "only" hast 16 threads so its a max. 8 core with HT or more cores and disabling HT (if the bios can do that), like when having a 12 core with HT
you might be better off with esxi or proxmox and using a dsm vm, that way you can handle resources more flexible (cpu power for other vm's) and the nvme's can be made virtual ssd's and 3617 can handle them as normal drivers or as cache - but you loose a lot of simplicity compared to a baremetal install
that sounds a little low for HA, consult synologys design guides for that, maybe plan a added 10G nic for the pcie slot (4x or better) and use a direct connection between the two HA units over the 10G connection (you don't need a switch for just connection the two units)
also check the forum if HA is available without a valid serial
xpenology is still a hacked dsm appliance and there are things not working without "tweaking" and you can loose functions when updating if synology changes things and if you can't install all security updates there are risk's involved like you cant install 6.2.4 that already contains new security fixes, also its easy to "semi" brick or damage the system on updates when doing things wrong (like not disabling write cache on updates when using 918+ with nvme cache or installing "to new" updates like 6.2.4) like when having other people not aware of the specialty's handling the system - keep that in mind)
have you given open media vault a thought (i guess they will have no equivalent to HA but webgui, nas and docker will be there too)
i don't know how close to production that system is supposed to be but it can be riskier to do that then you expect it to be (if you dont have longer experience with xpenology)