I think umbrel or start9os is a good start if you want to go the full node route, they both have the btcpayserver app. You can run it pruned, so no need for a crazy big hdd. The tricky part is more related to networking and proxying. I usually set up a clodflare proxied domain to the btcpay server instance, this is the most painful part of the setup. If you dont have a static ip it adds to the pain. But it really depends on the use case, for a farmers market you maybe don't need btcpayserver, maybe just a normal ln node + android wallet...
Discussion
so will a merchant need to invest in a server +($1000 or more) and become a sysadmin to take btc? is there an easier, faster way?
It really depends on the use case and the merchant, someone who knows a little bit of linux is completely different from someone who can barely use a smartphone. I think if they are just starting out
and expect 0-1 transactions per month a node might be overkill, a soft wallet would suffice. But yeah, depends from case to case...