+1 for baltic networks. not a scam, but also not a deal, just good price. mikrotik in general is quite late to multi-gig world, good that finally they've released something affordable for SOHO @2.5gig.
>I would, if possible, like a 20G link between my switches. Should I do this with the stacking functionality or some other LAG?
yes, go with LAG, LACP (unless you are stacking - then it may be unnecessary)
> How should the ISP connection be forwarded to the pFsense-box? IP-address is provided...
if you only knew how many enterprise deployments roll without any vendor support. Especially, if the vendor's support terms, prices and lifecycle is user-hostile(hi, cisco)
and regarding ECC/redundant-psu, from practical experience, the long-term networking hardware failures rarely come from these.