A slight correction in your statement.
The P365 series all uses the same width grip and slide. Both are slimmer than the Glock slim single stacks.
So if you go from there, the Sig P series is slimmer, holds more, comes with night sights standard, has a near better than most trigger, has 10, 12, 15, and 17 rnd mags that all fit the smallest grip with "x grips" for all sizes in the factory box, grip swaps are done in 30 seconds....if you asked is there anything that is an alternative to the P365 that functionally bests the P365, the answer is no. Honestly, if Sig started over on the P365 idea with a different grip for the larger sized mags or introduced like the P320 grip S/M/L/X grips, I don't think the P320 has much reason to exist other than Sig was lazy and used the failed P250 as the base. I suppose it's heading that way with the new AXG/Legion P365.
Slides, FCU, and grips are interchangeable.
I have multiple P365 models and shoot a bit. The only complaint I have is the factory 15rnd mag for the 10 round frame is very poorly done, but it functions 100%. The factory "x grip" filler doesn't cover the back of the mag on the 15 round grip extender. The 15rnd mag on the 12rnd frame with factory grip extender doesn't have this issue.
If you buy a Sig p365 now, the current line up is all with a "P" in the sku and should be optics ready. The P365 without an optic mount is now old stock. The P365XL with an optic plate that has the sights attached is old stock.
I am not a Kahr believer after owning and shooting them a lot. I was heavy only in Kahrs for a long while with 3 CM9s, 2 PM9 (black and silver), PM40, and a P380. My PM40 and P380 were total train wrecks. Kahr's mag is just such a contresy, but for sure no one can challenge their construction at the back is high school welding class at best. Also, Kahr changed their mag followers, then didn't, then did, but never told the public. I think that's pretty shady. Their extended +1 mags have some people on the net saying they work, but the majority seems to agree they don't. If you care to follow this, google on your own to see.
The smallest 9mm on the market is the DiamondBack DB9. It is on gen4 and still a total failure. Do not buy. I have had all generations and all have major issues with the recoil assembly. DB9 generations have all been significant generational changes, not minor cosmetic. None have saved it. I want this model to work the most of all micro 9mms. It's truly the size of the LCP.
Not that it's impossible, but I think HK and Walther doing the P365 sized gun better just is very unlikely. I will be buying both if they can do it though.
Just one person's opinion