A short overview of the composite upscaling situation:
When it comes to Composite, it'll always be a messy signal, and the TI 99's particularly bad. In most cases of computers and consoles which are limited to RF and/or Composite output, mods featuring superior output options are pursued where possible. Unfortunately (as likewise hinted), while there was such a mod for TI 99/4A, offering 31KHz RGBHV output (i.e., "VGA"), this is no longer available. And there is no immediate replacement, since the TI 99 is not a case like those of a majority of other systems, where there are preferable video signals to be hijacked off the board somewhere (so all you need is amplification, possibly/optionally sync processing, and a jack). It's a case analogous to the NES or 2600, where you either need to outright replace the PPU/VDP (TI99 F18A mod, NES PlayChoice-10 mod), or do some real time analysis of palette RAM interactions (2600RGB, NESRGB).
There are some devices out there which do as well as may be done, nonetheless, in processing, upscaling and even sometimes cleaning up the idiosyncratic and non-standard 60Hz-per-scanline every-other-scanline Composite video produced by analogue era video game consoles and some early home computers. Micomsoft's XRGB line of scalers (from Japan) are the top of the line, as far as that goes. The market for this sort of thing is otherwise dominated by line-doubler solutions such as the OSSC and RetroTINK. But because these don't do framebuffered image processing, they can't really do much of anything to "clean up" a bad signal. They're simply duplicating (or triplicating, or quadrupling, in the case of the OSSC) scanlines. Moreover and in any case, the OSSC does not accept Composite input, for the simple reason that this is not in high demand, among the enthusiast community (due to its poor quality). So this makes the meaningful options, when it comes to upscaling video like the TI 99/4A's Composite signal:
RetroTINK: A line-doubling, transcoding solution which is, as such, very light on options and picture processing, but gets you to HDMI 480p, for what that's worth.
XRGB-3: A relatively powerful upscaling and transcoding solution with both framebuffered and line-multiplying modes. Accepts most signal types, including Composite. Significantly, outputs both analogue and digital.
XRGB-Mini: A feature-rich framebuffered upscaling and transcoding solution. Accepts most signal types, including Composite and, also, HDMI. Easier to use and more of an "it just works" solution than the XRGB-3.
There are also devices designed for upscaling regular interleaved 15KHz broadcast signals, which can be found. These incorrectly process the non-standard signal as if it were interlaced, the principle effect of which tends to be that every other frame is dropped. Some noticeable latency is also typically introduced.
In my opinion, when it comes to Composite output, you're better off just using a CRT. To me, even using a higher end transcoder, digital display just exaggerates issues. Where a CRT really helps to smooth out the noise.