I've read through what a few others have said and I hope you dont mind someone else chiming in as well-- It started out as a comment reply on the post but it got too long ;;
Alongside being a retelling, Stampede, or at least S1, is technically a kinda-sorta prequel sort of situation, since the final event of Stampede is something that happens before the main story of '98 and the manga.
The characterisation in Stampede isn't exactly inaccurate to the manga per se, but since it is a retelling that recontextualises and restructures a ton, it's definitely not 1:1. But for that matter the 1998 anime is also not 1:1 with characterisation, and I'd argue that it's just a bit farther off due to some of the comedy choices that were made, which were rather OOC in comparison to the manga, and also because of some of the plot changes due to the manga not being finished, as you previously mentioned.
Many of the more serious themes present in the manga also come through much stronger in Stampede, so far, than in 1998 despite the greater deviance from the manga's plot right off the bat. But I would say the first part of the manga is a lot easier to read through if you've watched 1998 first, since beat-for-beat, there's generally more similarities between the two, which makes sparsing out what's happening less of a task. The manga's pacing however, imo, is actually more similar to Stampede's (fast, a lot happening) ironically. 1998 has a lot of filler very early on which makes it the odd one out in terms of pacing.
The manga is definitely darker than the 1998 anime, and Stampede seems to, so far at least, not be shying away from those darker elements. While there is some of the same fun in the first portion of the manga as in 1998, the latter portion is very much a seinen, and Nightow takes full advantage of the ability to explore more mature themes that being under this demographic label offers.
Also, as a side note, some of the production team for Stampede actually hadn't watched the 1998 anime prior to making Stampede, and had only read the manga, which is part of why there's such a stark tonal difference between the two anime specifically. It's very hard to pit them against each other in any definitive way, so I definitely second that it's worth experiencing each iteration!
YEAH!!! i really appreciate it this!!!! yeah, i know fuck all about what the manga has and doesn't have cause ive been trying to avoid as many spoilers as i can for it so this is really helpful, i didn't know that the characterization is so different within 98 and the manga, that's gonna be fun to explore lol, also that the tone was pretty different! i know trigun is lighter than trigun maximum so im also interested to see how that plays out
and also yes! i do know stampede is a prequel, retelling, sorta thing,, it just rolls over a lot of the character introductions and like, early storybeats within it's prequel to the main storyline while also including flashbacks and shit that happen later in the 98 anime so it's confusing to classify
thanks so much for your input! i really appreciate it cause i have no clue abt this stuff, again, haven't read the manga yet :)