Upgrade my CD player or futile effort?

Cheap enough to try a DACS…https://www.topping.audio/productinfo/645180.html

I had an Ares II and was happy with that but dove a little deeper and got the Pontus II; didn't see what your budget was (will assume you're "dipping your toe in"), so there's a lot of people here that will offer more expensive and better quality options here than what I'm suggesting.  There's a $1000 jump from the Ares II to the Pontus II but I felt it was worth it, but I stream heavily and use my CD transport when the mood strikes me (Cambridge Audio CXC V2; had the Audiolab one but preferred the CXC) .  I would second the suggestion that you get a DAC that works for you feature-wise and hook up your Marantz to that first - only change one thing at a time, and it might actually be the DAC in it that's poochy.  There's a bunch of good DACs out there for whatever price point you're at so it's unlikely you're going to get a dog.  Any Denafrips DAC are going to be the ones with the least B&W so no bluetooth, remote, or wireless capability, and if you buy directly from Vinshine there's no return policy if you don't like it.  I've had a number of Topping products and they've all measured well, were packed full of features and sounded good but never moved me enough to keep them.  I hear many raves for the Jay's Audio transport but we're talking about $2400 and it's a top loader so that has to be accommodated.  Alternative: the Bluesound products have an OK DAC built into them and allow you to get into streaming at a low cost; requires you install their BluOS software which works very well (I think) on either your smartphone or computer - no fees or subscription needed.  You can stream internet radio for free or play any flac files through it, and you can always bypass the DAC in it if you get a better one and use it only as a streamer.

OP, you stated "Obviously now I know going with a better CDP without a DAC is worthless." Not obvious, and not true.  I recently plugged my Oppo UDP205 into my den (main) system; it had been anchoring my home theater since purchased (ca 2017) on a system that was more pro and less audiophile.  My den system had been using either a Sony S9000ES SACD player or Pioneer Elite BDP05, both of which were about 20 years old (and consequently 20 year-old DAC technology). Holy smoke!  Soundstage, frequency range and dynamics galore!  I know a lot of A'goners bitch about the Oppo DAC, but the point is, a good quality CD/SACD player runs circles around older CDP's, and you shouldn't discount them.  Look at current models from Luxman, Marantz, and Technics, to name  few (which should set you back $3K-4K).  Or you can try the separate DAC route, but try to avoid the optical connection if you can help it.

FWIW I am waiting on Esoteric K-05XD and the K-07XD SACD players which should be released in a couple of months.  Rave reviews for the K-01XD and the K-03XD which have been out for a couple of years now.  All the Esoteric models have among the best tray mechanisms in the world, and the new "XD" versions have a much-improved DAC.  So you get a phenomenal transport and a fantastic DAC all in one.  They have all kinds of digital inputs to use the DAC in the player.  For me, it's a no-brainer upgrade.  But first I'd like to demo when they come out.  Guessing in the $7500-10K range, so not cheap.  But again, it's a player-- 2-in-1 transport and DAC that has USB input, among other inputs.  And they play SACD's.

Whether DAC or new player the question is what makes them sound better than your modest unit?
Answer: Internal analog amplification circuit, power supply and filtering capacitors. Look under the hood.
I have a Marantz SA8005 SACD player and if you look for online pics of the chassis you will see what I mean.
The analog amplification circuitry gives the sound its drive, dynamics and energy as well as freedom from noise. The SA8005 does this quite well but is out of production.
The newer Maranttz SACD players do this well too.
I also have a Luxman D05 which uses an R core transformer power supply, the best IMHO. Lower noise and improved dynamics.
As others have mentioned The D03X is an excellent sounding player.
I prefer a one box solution. However there are some excellent DACs with robust power supplies. Look under the hood at online pics.  Avoid DACs or players with empty chassis.