Buy a 350 as a semi-beginner?

I didn’t go from a 250f to a 350 but I will say the 350 sounds like a great bike for you. It’s not so powerful it will scare you but it will have noticeably more power everywhere over the 250 and they are very friendly bikes to ride. I’d say go for it. 

ask someone who has a newer 350 to ride it once 
I had a Husky 350 and wound up selling it because I like YAMAHA YZ250F a lot more 
 

Just keep ypur 250 and ssend the cash on tyres, racing and gas

350s are easy to go... out of your skill level fast very quickly

the 350 is worth it just to upgrade to a newer bike IMO.. the 2016 250 has those horrible 4cs forks right? or first gen air?  Either way the suspension will be better and a 2016 250 is getting a little long in the tooth, sell it while it's still worth something.

you can't go wrong with either, If you can find a good deal on a low hour version of either, go for it, I would personally lean 350, but nothing wrong with the 250.

Unless your just itching for an upgrade or the 250f you have is in rough shape id ride that thing for another year and really see what you like. Those were great bikes and are far more capable than what any of our skill level will ever be. 

I have had both recently sold my 350 but I still have my 250 (also have a 15.5 450sxf) but the 250 is what I kept as my main track toy.  to me the 350 felt closer to a 450, you could be a little sloppy on the gears, and the throttle control in the corners is closer to a 450.  

Ironic coming from a guy that has "yz450" in his username.

Ehh not abt what i ride, this guy said he doesnt ride to often and has limited moto experince, dont kid yourself modern 350s are easy to go fast on, could get in to trouble quick, I'd stick with a new 250f

Are you riding in deep sand? If so the 350 might be easier to ride than the 250.

Get the 350.

The only people that ever regret a 350 are the few folks who truly enjoy and can use a 450....which is not very many of us.

the 23+ 350 SXF/XCF motor is quite a bit more powerful in the lower and mid RPM than previous 350s.  You can add the CUO tool and make some really nice mild maps and upload from your phone in less than a minute and also go back to factory settings quickly depending on track conditions.  I have my 24 tuned for less engine braking and smoother roll-on and it's awesome, also doesn't stall as much.  Funny thing I noticed that when reducing the throttle response, the bike also seems to feel "lighter" and more similar to the 250, but with more torque.

I have bounced around bike sizes and am also a vet currently on a 22 Husky 350. Honestly, the biggest different between a 350 and a 250F is just an insane amount of top end and over-rev. A little more torque on the 350, but that also would depend on the brand since some 250Fs have more torque than others. Handling will be very, very similar, which is a good thing. You said you ride sand, so the extra top end HP might come in handy. 

If nothing else, the forks on that current 350 are going to be much better than the 2016 forks unless someone has made some significant changes to that bike before you got it.

That's a question that only you can answer.

I liked my 350SXF(2018) but I wasn't in love with it.

At my skill level I had a lot more fun riding the YZ250F than I did the 350.

To me it was more fun being able to ride the 250F WFO than it was 350 at part throttle just because of how fast those bikes actually are.

2017 HUSKY 350 
It was MINT 
Just the 2021 YZ250F is SOOOOOOO much fun to ride 

You’ll be fine, don’t be afraid to put it in mellow mode, or add an extra filter sock or two, it will mellow it out drastically lol.

I bet you’ll love it as is though especially in sand that takes it down a notch to begin with.

Id get the newer 350. Good choice, you will enjoy it!

I felt pretty unsure about my 350 when I got it. The suspension felt so strange and unpredictable. I did the KYB swap using @cwtoyota guidance (he owns Dialed MFG). 

Then I threw on a REP linkage and it made the bike feel like a dream to ride. 

I’ve never loved a bike so much as I love this thing now, it makes me want to ride all the time.

The power is insane though. No shade to anyone, but I think a lot of people on here are kidding themselves when they say they're wide open on the throttle on any bike they ride. This bike rips to the moon (I’m 175lbs) when in the right gear and you’ll definitely need to take your time to learn it. 

You’ll have a blast on a sand track

I fully agree and thanks for the shout out.

Fork upgrades and REP linkage are a great combo on these bikes.
I raced that setup with a Honda / Nissin front brake last week at the Washougal amateur days and I'm very happy with it.

For a beginner, MAP 1 would be a great place to start.  The traction control mode takes even more of the edge off.
The 350F might be easier to ride than a 250F if you ride lower in the rpm range since the 350F has a bit more torque and requires less shifting.

I've added a Vortex with XPR maps to my GasGas.  When I run it in the mid/high RPM this bike makes more than enough power for me.  I'm 165lbs and racing 40A class.