I've used several over the years. Honestly, for swapping between different iPads like your 2018 Pro and the newer Air, you need something stable. Being quite satisfied with a couple of options, I can say they haven't given me any connectivity headaches.

• Logitech Crayon Digital Pencil for iPad is incredibly reliable. It doesnt require pairing, so you can basically just turn it on and go. The lack of pressure sensitivity is the only real downside for design work, but it never drops the connection.
• Zagg Pro Stylus Dual Tip with Tilt Support is another solid choice. Using it across different screens works well and the dual tip is handy for navigating menus. Personally, the Logitech is better for a tight deadline. It feels sturdy and Ive never had it lag during a project. It definitely feels safe for professional use.

To add to the point above: since you're in the middle of a design project, you should know that almost every third-party stylus lacks pressure sensitivity. Its a hardware protocol thing. If you really need that feature without buying an Apple Pencil, the Adonit Note+ Professional Stylus for iPad is one of the very few that actually supports 2048 levels of pressure and tilt. It connects via Bluetooth and charges with USB-C, so it works fine with both your 2018 Pro and the newer Air. If you just need something for line work and don't care about pressure, ESR Digital Pencil for iPad with Magnetic Attachment is technically solid. It uses active capacitive tech so there is zero lag, and you dont have to pair it in settings. You just tap the top and start drawing on either tablet. Its super reliable for a backup and way cheaper than the name brands. Just bear in mind that for shading, you'll be relying on the tilt function rather than how hard you press.