The Honda Rebel 500cc would be more similar to the Harley you have now.

The MT-03 will feel sluggish for sure, go for the MT-07 instead. You can (and should) find aftermarket seats, all new bikes come with crap seats that give you a 10m grace period to get to a motel before s/he quits.

If you’re into switching to nakeds, you can’t go wrong with a Honda Hornet (CB 750), Suzuki GSX or Yamaha FZ6 (the latter is my bike), all 600cc/100hp. Most other nakeds are 60hp. All accessible in the used market.

You’re used to the torque of a V-twin 750. A 300 will feel quite lumbering comparitively.

Also, having a passenger on that 300 isn’t the most fun, your Harley would handle a pillion better (though not much better in stock form), since the seating position isn’t so high. Neither bike in stock form is really meant for a passenger, but the combination of far less low rpm torque, the high center of mass from the seating position, and the higher rake angle of the Yamaha will make controlling it a lot harder (I know, I’ve had both types of bikes, my sport-tour has more power and a lower center of mass than this Yamaha, and the rake angle alone makes having a passenger challenging, this with 40 years of riding behind me).

But, the Harley would be more comfortable for a passenger with little more than a new saddle (a more pronounced king/queen) and a small back brace - Harley even sells one, about 8" tall (so kind of unobtrusive).

The back brace will give a pillion a bit of security, an improved saddle will make it less cramped by lifting them away from the foot pegs.

(I’m not even a Harley rider, never owned one, don’t like them, but I’ve ridden them and understand why new riders and people who ride two-up go for them, the torque and low center of mass are ideal for both groups).

Keep the Harley.

This is a common discussion for folks who have decided on a Honda Rebel, but can’t decide 300, 500, or 1100. 300 is no or very low highway riding, it can do it but it’s not enjoyable. 500 is for some highway riding, still a bit light and you can feel windswept, but good economy. 1100 much more planted and powerful and very capable of long trips, but a lot more expensive.

Any particular reason you want to change? If it’s just to ride a different style of bike, is it in your finances to get a used mt03 to putt around in, then sell once you’ve had your fun?

I have an 80s 450cc, daily commute on the highways. She can do more than 80mph, but kinda struggles to get there past 75mph.

I mention this because I observed my former buddy with an mt-03 having similar limits when we rode from San Francisco back to LA on the I-5, and of note is that I run 87, he had to run with 93 to get that performance.

I’d hesitate to get something that small. There’s a 400 ninja and some other options out there, but I’m considering a Diavel, an xsr900 or a 900rs cafe.

Take a look at the used bike market near where you live. You will be able to find better deals on something you might be interested in.

If you want another cruiser, you should go for something with at least 650cc for adequate highway power. Note that many cruisers, especially Harleys, can be modded extensively.

If you’re looking for a naked bike, pay attention to the 2nd seat, as many have tiny uncomfortable passenger seats. I’d also advise you to get at least 400cc for adequate highway power.

Another category of suggest if cruisers aren’t your thing is a sport touring motorcycle. They have lots of power, are fairly comfortable (even for passengers), known to be very reliable, but can be a bit heavy. When new they are pricey, but in the used market you can find them for under $5k.