Depending on your SSD TRIM may not actually be required anymore. If you're running on a VM it's even less important as the disc is likely to be virtual anyway.
That being said the filesystem certainly is the weakest part of the OpenBSD and given the uptake in filesystem designs with ZFS, Btrfs and Bcachefs it is interesting to see that OpenBSD is left behind.
For a personal mail server, or even small business, it's not really an issue, you're likely not going to have terabytes of email.
It has certainly worked very well as a server OS for me the past 20+ years. I agree that FFS on the whole is a very dated and sluggish file system, though anecdotally I've not once suffered a loss of data with it despite several power outages and sudden hardware deaths. It may have mattered that I rarely ever had "softdeps" enabled on my file systems.
I love OpenBSD, have been a user (including professionally) for over 2 decades, and appreciate their stubbornness when it comes to security (including sacrificing performance), but for work for anything other than a layer 3/4 firewall I'd use FreeBSD (FreeBSD has an older, threaded version of PF, though). It's got ZFS for storage, a much more robust threading system (meaning modern multi-core processors will be better taken advantage of) and generally has broader support for hardware.
Pre-2005 running FreeBSD was a nice little "secret" that allowed you to run a rock-solid OS without drama or headaches. However, professionally nowadays I've accepted the fact that linux "won" and I don't want to deal with the headaches of finding people that can admin something niche, on top of so much tech tooling being designed on and around linux. Most of my work is done supporting docker containers in some way, so why fight even if it's possible to run docker on FreeBSD...
That being said the filesystem certainly is the weakest part of the OpenBSD and given the uptake in filesystem designs with ZFS, Btrfs and Bcachefs it is interesting to see that OpenBSD is left behind.
For a personal mail server, or even small business, it's not really an issue, you're likely not going to have terabytes of email.