Best investment; cartridge or line stage?

Replace the Cartridge.

Existing cartridge is an Elliptical shape.

Stylus Shape

Select a cartridge with an advanced stylus shape: SAS; Shibata; Line Contact; Microline; ... (other variations) providing greater contact with the groove walls: better for fidelity, less stylus wear and less groove wear. Extra Cost: when advanced stylus shape’s longer life is considered, is not such a big difference.

Cantilever Material:

greater stiffness relates to better performance, and in my experience improved bass. Many cantilevers are aluminum, I prefer Boron. Harder materials, typically crystals, cost more than I want to spend. The Sapphire I own and past crystals, if you concentrate .... but they have not been obviously better. However the beryllium cantilever of the Shure V15Vxmr was the best bass I ever had, obvious to all. The Thorens TD124 TT was also instrumental for that.

Imaging:

ALL Imaging is Phantom, therefor always check to see two factors that help provide better Imaging: greater channel separation and tighter channel balance. Not just overall image width, more importantly the combination reveals the location of instruments and vocalists more precisely, everywhere, all with greater distinction.

I look for 30db separation combined with 0.5db balance.

Sound Characteristics:

Prefer, not Better. Aside from Imaging, the sound characteristics are subjective, people describe what they prefer, often calling it better, but ....... This is the hardest part when finally selecting.

Stylus Life:

Note: stylus life expectancy in this article is based on technical specifications from Jico: actual life is much longer, the life expectation differences based on stylus shape remain relative

https://www.sound-smith.com/articles/stylus-shape-information

Alignment Skills and Tools:

A few inexpensive tools, and acquired alignment skills are needed to mount any cartridge: new you install or eventual replacement of OEM pre-mounted cartridge. You, a friend, a shop: watch, learn, practice, it’s about being careful, not as difficult as many people think. Removable headshells are much easier to work with than installing cartridges on a fixed arm.

Mad Scientist Contact Enhancer can work wonders when all connections are treated. Cartridge is the likely better upgrade. Be sure to treat the ends

HTH

I think this is the wrong question!

The reason is simple: the tonearm's ability to properly track the cartridge far outweighs what cartridge you have. The better the arm is at this, the less differences you'll hear between cartridges assuming they are set up properly.

(FWIW Dept.: I use LPs I recorded as reference. This is a very useful tool which I recommend to anyone who wants to create a proper reference; being there when it was recorded is valuable for knowing what how its supposed to sound.)

Once you have that nailed down properly then I'd look into the phono section. A bit of a teaser tip: if the phono section has a 'cartridge loading' switch for LOMC cartridges, the designer is likely unaware that the 'cartridge loading' resistor is really for the benefit of the phono section rather than doing anything beneficial for the cartridge (it impairs its ability to track properly). There is a simple phenomena at play that you learn about in the first week of electronics at either the technical school level or college level. Something to think about...

@elliottbnewcombjr @ghdprentice thanks for gleaning my questions' true aim here. I've had a few discussions with dealers and cartridge was the overall suggestion. I like to check in with my people (aka audio heads) before I make these kind of decisions. Elliot I appreciate the link.

@tweak1 Another cartridge vote. Thanks for chiming in. And thanks for the reminder about enhancers, it's been a minute.