You have to remember that all the TV master tapes (both NTSC & PAL) were created in the 1990’s. And, unlike SeaQuest DSV or Babylon 5 where the producers had to deliver film masters (and in the case of SeaQuest, the film masters were used for the International PAL/SECAM masters, while the NTSC masters were the rough cut versions that went to air, and now SeaQuest could be remastered into 1080p or even 4K), the producers of DS9 and Voyager were not required too. So the only transfer the film had was to 480i 30-frame-per-second NTSC D2 Composite Video tape (and we are talking true composite video, not Y/C composite like VHS or S-VHS where it would split a composite signal into a color and separate black & white channel on the tape, but composite like Laserdisc), unless it was model/sfx shots, then those went on D1 NTSC 30-frame Component Video Tape to preserve quality during all the generations of compositing that were required. (And later on, the CGI shots were output to D1 NTSC tape.). However, then everything from the D2 and D1 tapes were edited together on one D2 NTSC Composite videotape and that NTSC master became to basis for ALL NTSC/PAL/SECAM tapes. And in 1994, 60% of stations in the USA got their copies from a satellite feed that they then copied to whatever video tape they used (Betacam SP, S-VHS). The other 40% got their copies on 3/4” U-Matic. In Canada it was probably the same amount, however in other countries like the UK, they might’ve got their masters from the satellite (which would’ve been sent in 480i) and the station would’ve handled the conversion themselves or Paramount would’ve run them through a machine that was specially design to convert and upscale the NTSC 480i 30-frame to PAL/Secam’s 576i 25-frame (and in the case of SECAM, they probably used the PAL master to create the SECAM master by running it through another special box that was designed to convert the PAL color into the SECAM color).
And all of this conversion would’ve most likely been handled in the analog, composite domain, even if they were going from D2 NTSC to D2 PAL, since they were most likely using equipment from the 1970’s-1980’s to do the conversions (D2 tape was originally created with the intention of being able to slip it into analog set ups without having to spend millions to change everything over to digital to work with D1 properly). And stations that air DS9 & Voyager in HD are still using SD tapes from the 90’s and upconverting them to 720p or 1080i. As for Paramount plus, maybe they upconverted the files to 720p and they are streaming them in 720p and using that to make them look a little better at a slightly higher bitrate. For years, even on broadcast TV, because of compression issues, 720p has generally been considered equivalent to an upconverted DVD. 720p is usually broadcast or streamed around the 11-to-13 Mbps at a variable bitrate. DVD’s max bitrate is 9.2 Mbps, with most DVD’s are encoded with a variable bitrate in the 5.0-7.0 Mbps range.
So, in order to get better video for DS9 & Voyager, both shows need to be re-edited and have the camera negatives rescanned in either 1080p or 4K and edited that way. As for the current files, they were made 20 years ago and were using 3-D comb filters that were top-of-the-line back then, however you can get better color separation on your modern TV’s (if they have a yellow composite connector or in the Uk, the composite SCART connector) by connecting your DVD player by composite. It forces the video back into composite and then your modern TV’s 3-D comb filter will give you a better color separation. Another option is to track down the DS9 & Voyager Laserdiscs that were released (as these are just analog composite copies of the digital composite masters, so they never went through a separation circuit) and connect a Laserdisc player by composite and you’ll get the current ultimate video that way.