Would you stick with Fortinet as a solo sysadmin?

I would not replace a working network just because the interface is unfamiliar. FortiGate managing FortiSwitch can be convenient for a small team, and a full stack can be good value if the firewall is sized for the job.

Use the new building to test the current approach against your actual requirements. Check the coverage and capacity you need, then compare the cost and management work of FortiAP with another wireless option. You can make a gradual decision rather than committing every site at once.

Keeping FortiGate and changing the access layer is a perfectly reasonable middle ground. Meraki switches and APs are an option if simpler cloud management is worth the higher price. UniFi APs are another lower cost option to evaluate, with FortiSwitch left in place.

Compare licensing and support costs as well as hardware. Also account for having another management system to learn and maintain. Mixing vendors gives you flexibility, but it is not automatically less work.

I'd treat security and patching as a separate decision from whether the gear is pleasant to manage. Fortinet vulnerabilities are a concern for some admins, while others value the vendor's acknowledgment and patching response.

Review how quickly you can apply updates and recover if something goes wrong. Keep management interfaces off the public internet and scrutinize the replacement VPN design. If that upkeep is too much for one admin, include other firewall vendors in the comparison rather than assuming the existing platform has to stay.

Figure out the budget before falling in love with a replacement stack. Palo Alto and Juniper Mist are alternatives worth looking at, but a preferred product is no help if the municipality cannot afford to buy and support it across all those buildings.

Build a replacement schedule around end of life, the VPN change and the new site. That gives you a way to compare a phased migration with staying put without turning it into one enormous solo project.

I would investigate the Wi-Fi complaints before making them the reason for a wholesale switch. Some reports describe recurring disconnects and firmware trouble with FortiAPs, while others are satisfied with the full stack. Check whether your own sites have repeat issues and whether they point to coverage, configuration or software.