Many residential ISPs won't give you a static IPv4 address.
Most residential ISPs won't give you sufficiently symmetrical bandwidth.
VM hosting companies have spare hardware on hand, and can generally shift you either transparently or with minimal downtime to different hardware.
If none of these things are an issue for you, go ahead.
Personally, I wouldn't use RPi hardware, EVO 970s, 2TB spinners, or a Fortigate.
Four boxes, each with a 12 core CPU, 64 GB RAM, a 2TB NVMe with at least a 1200TBW lifetime, and a pair of 4-8 TB spinners in RAID1. One box runs staging including workers, one runs production including workers, one runs all your firewall, utility, administration and backups, and the fourth is configured and powered off -- once a month, power it up, bring it up to date, and shut it down. Someday you will need it as a spare or for capacity. Get extra NICs for all of them.
I would estimate $5K in hardware costs. Buy an online UPS and connect the production and utility boxes to it.