@gabe-a my man, what I posted was from the Netgate website itself, but it only applies to in place upgrades for TNSR itself. If you read the documentation that @Derelict provided, it shows you how to update the underlying linux distro. It's not particularly complicated.
I am going to be honest with you, I've set up and deployed TNSR on an SG5100 multiple times at this point and have not had the experience you are having. The documentation got me up and running and I've been satisfied with my experience and the product - and it's only gotten better over time. My config isn't insane, just some VLANS, some ACLs, some dst NAT rules, and an ipsec site to site, though I see that Wireguard is now supported for site to site and remote access, so I'll be playing with that here very soon. If TNSR isn't for you, I can respect that. No one said we all have to like or appreciate the same things. I do think you will be truly pressed to find any consumer-level gear that can perform at the scale you are looking at. And anything commercial is likely going to cost you much more if you can get your hands on it at all.
To be very frank, you made a purchase based on a Comcast tech's advice without seemingly truly understanding the performance of the software for the hardware that you purchased. I personally would have contacted Netgate to discuss your performance needs and allowed them to help you select the best hardware. I believe the next option up (1531 or something like that) has the power to provide the performance you want/need using the more simple or intuitive PfSense. Netgate posts pfSense performance numbers openly and fairly and frequently stresses that imix is the more realistic test for performance.
I don't know what more to say or do to help you as this has become more of a rage venting than an inquiry for configuration assistance. Happy to try to be of help if the conversation goes the latter direction again. I also believe you could reach out to Netgate for advice or possibly even some configuration assistance.
EDIT: please do realize that I am not associated with Netgate, don't collect a dime for making plugs or promoting their hardware or software. I am not even formally trained or certified as a sys admin or anything like that. 'm just a regular user with a curiosity for networking and a desire to learn always.