Honestly, you gotta check out the Viltrox primes! I absolutely love the Viltrox 23mm f/1.4 AF Fuji X for low light. Its super fast and the AF keeps up perfectly with the X-S20 tracking. For portraits, the Viltrox 56mm f/1.4 AF Fuji X is a total steal under 300 bucks. These are amazing for the price and way better than XC kit lenses in the dark!

Like someone mentioned, those old XC motors are basically ancient tech. Its a letdown how much they hold back the X-S20. For your sisters engagement, you gotta prioritize reliability. I've had issues with cheaper third-party AF consistency in the past... you dont want to be hunting during the big moment.

• Fujifilm XF 35mm f/2 R WR is the safest bet. Super snappy, weather resistant, and used prices are way under your 400 budget. It just works.
• Sigma 16mm f/1.4 DC DN Contemporary Fuji X is solid if you need wider shots. AF is way more confident than the XC zooms. Avoid the TTArtisan 27mm f/2.8 AF for this. It was not as good as expected and the AF is kinda noisy/slow. For a one-time event, just grab the Fuji 35mm f2 and play it safe.

I've been geeking out over the X-S20's new autofocus logic and honestly, it's a massive upgrade, but those old XC lenses just cant keep up with the burst rates or tracking speed because the motors inside are too slow. I've been super happy using some of the newer glass that actually have modern stepping motors. If you're doing an engagement shoot, you need something that wont hunt when the lights go down. Here are a few that have been working well for me:

• Samyang AF 75mm f/1.8 Fuji X
• This lens is a total sleeper. Its incredibly sharp and the f/1.8 aperture gives you that nice subject separation for portraits. The AF is way faster than the old gear-driven Fuji stuff.
• TTArtisan 35mm f/1.8 AF Fuji X
• For under 200 bucks, its crazy how good this is. The stepping motor is quiet and it keeps up with the face tracking perfectly.
• Fujifilm XF 50mm f/2 R WR
• If you find this on sale or used, definitely jump on it. It uses a linear motor (LM) which is silent and basically the gold standard for speed in the Fuji world. Basically, the X-S20 needs lenses with high-speed STM or LM motors to really shine. Your old XC lenses use DC motors that just dont have the torque for the new phase detection points. Stick to these newer primes and you'll be fine for your sisters big day... just make sure to update the lens firmware too if they have a port.

The X-S20 is a beast but it really shows how slow those XC lenses are in the dark. For the engagement shoot, you definitely need faster glass tbh.

• Sigma 30mm f/1.4 DC DN Contemporary Fuji X is probably the best value. Snappy AF and f/1.4 helps a ton.
• Fujifilm XF 35mm f/2 R WR is a decent option used. Much more reliable than the XC stuff.

@Reply #4 - good point! Honestly, the old XC glass is definitely what is holding you back. I have been super satisfied with the AF performance on the X-S20 once I swapped to glass with better motors. For an engagement shoot, you really need that reliability so you dont miss any candid moments while the lens is busy hunting. Since you have a strict budget, here are a couple more options that work perfectly with the new tracking system:

• Sigma 56mm f/1.4 DC DN Contemporary Fuji X
• I cannot recommend this enough for portraits. It is incredibly sharp and the autofocus is nearly instant. It usually sits right at that 400 dollar mark or less if you find a deal. It gives such a nice background blur for those romantic shots.
• Fujifilm XF 23mm f/2 R WR
• If you need something wider for group shots or environmental portraits, this is a tank. It is way faster than the XC kit lenses and the weather sealing is a nice bonus if the weather turns during the shoot. You can easily find these used for around 300 bucks these days. Both of these will feel like a night and day difference compared to what you are using now. Good luck with the shoot!