Dream job not exactly a dream. Managing unrealistic expectations.

After a few months of unemployment, I am now working at a place that I would consider a dream job. Growing company, I own the infrastructure, major growth opportunity and equity. The place I want to be at this point in my career. The atmosphere is great. Lots of long-term employees. Owner is fairly tech savvy; he did it all when he started the company and still has an eye on things, and that's where the problem is. I replaced the gateway, and a switch that was not properly configured by another person, failed. I swapped the gateway back until I resolved the switch issue. Now the gateway is fine. We have four IPsec VPN connections; one live, one a live backup, one to our SD-WAN provider, and another to a test location. User's PC is crawling. 16GB of RAM, 256GB SSD. Has been online for four days, but that's really nothing, as some of our computers may not be rebooted for two or three weeks outside of an update that requires it. Only really uses VoIP app, Excel, and Chrome. No Outlook, only Webmail. AV software, remote management agent, Three times in a week, the user has an issue that required killing a process or rebooting. Meanwhile, coworkers with slightly newer computers performing all the same tasks, but with 32GB of RAM instead of 16GB are not having any issues. Solution to me was more RAM. Problem is that this PC was the only one we had no stock of parts for. Instead, I swapped it for another PC from storage, which I combined parts from another stored PC to give it 32GB of RAM. There were no issues swapping over the existing SSD. Once past the bitlocker prompts, a driver and BIOS update and the machine worked fine. Total downtime for user was shorter than their lunch break. Owner has a problem with this. I didn't look for the why of the machine lagging and crashing. Looking into the logs and finding and correcting for a cause, assuming I found one, would have taken far longer than 20 minutes it took to swap the machine. Assuming the problem IS the lack RAM, and all signs pointed in that direction from my brief diagnostic, this issue SHOULD be resolved. Isn't the goal to get the user back up and running ASAP? Should I be spending more time looking for a problem that may have a more complex solution, rather than enacting an expedient solution? I need a way to tell ownership this without sounding like I'm being insubordinate. And then there's the desire for a period of time with "zero issues," including things that are outside of my control. Sure, there is acknowledgement that I can't do anything if there's a power failure, a network outage, or if one of our cloud apps has a problem. But no issues? I can't control user actions. I'm not permitted to force reboots to ensure computers aren't online for weeks at a time. I can't do anything if Kelly in accounting moves her laptop and breaks a device port. Somehow, though, those are issues that I am responsible for ensuring do not happen. Ownership isn't unreasonable, and they'll listen...but my first two weeks on the job consisted of Microsoft shitting the bed, a major cloud application we rely on having issues with their own connectivity (that they denied until shown otherwise) and the misconfigured switch killing half the office. And I admit that it was my own hubris that made the switch an issue. That is certainly coloring their opinion of me, despite their frequently stated faith in my abilities.