RE: Goldbox
Radiance is a personal favorite of mine, but it doesn't let you enjoy certain aspects of goldbox combat (such as backstabbing) as well as Kyrnn, or Azure BLades, because the scaling number of enemies means that you have to fight down the WALL OF 20-40 orcs and thus you don't get that freedom due to the pressing concern to armorclass and to hit bonus your way through and then sleep/stinking cloud over experimentation with other spells. I do like it's maps and clearing it block b y blyock, but it prevents me from recommending as a starting game. It also lacks the pseudo cursor memory or fix command (in other games using rest will automatically assume you want to memorize the spells you chose last time, but Radiance is all manual all the time)
Frontier is also an okay starting point in theory, but I think it's a little TOO easy, even for attempting the first in your sequence.
At the end of the day Kyrnn or Azure Blades is probably the best starting Goldbox game, because enemy quanity is toned down in favor of quality, which makes backstabbing feel more reasonable to pull off, as well as making you feel more open to experimentation in general. (It has higher highs combat wise as well)
Buck Roger's has a couple serious differences that make it a different experience than other GB games -
1: The medic skill occurring post combat automatically kind of forces the player to have the benefit of what would be "rest abuse" in other games, while technically being clean.
2: Grenades and such act like the glorious fireball spell of the other games, but without slot limits, the lategame can lean to totally nuking encounters in a way you can only do on special occasions in other GB games. I do like the protective items like flak, especially comparing lategame silverblade/pool of darkness, but that won't happen t you in Azure and Kyrnn anyway. I think playing it first leads to an impression of GB games having "actives" used more frequently than they normally are and it would be weird to go into Radiance or Kyrnn second and suddenly realize how much bigger the vanilla fight command is.
As for the rest of the games.
I played Legend of Grimrock before first playing Eye of the Beholder - HATED (grimrock) and went into Eye of Beholder with low expectations, I would say I ended up with a medium-average reaction to EoTB with things I liked and disliked (but that's so much better than how Grimrock treated me)
M&M is easy to get into. It's probably a safe choice for first old RPG. I personally would go Wizardry first though.
Wizardry 1 is a pretty magical experience, the game is really tightly built, which works to the strength of playing it for the first time, even for someone new to DRPG. 2-3 do use the same 20x20 grids, but don't have extra QOL improvements to really make playing them before Wizardry 1 have any purpose - I feel like it's the only one of all these series where it's possible to play the later games first and then go back to the older games and STILL have a rising, more and more positive experience.
Wizardry 6-7 (especially 7) score very highly with their fans, but are probably the most unforgiving starting point - It might be worthwhile though, just for the sake of arguably the highest high points of all of these games.
Wizardry 8 - I actually played it first of all wizardries and I loved it < It doesn't really naturally lead back into the other old school games and you can feel like it's trying to distance itself mid playthrough though.
Ultima...……………. Is not for everyone and certaintly not me (I think Ultima Underworld > rest of Ultima franchise)
Ultima 1-3 are kind of skippable even if looking for a history dive.
Ultima 4 is the first one that is a major historic milestone - Ultima 5 is much more playable and an equally good time capsule of thematic world building elements beyond the fighting part of good vs evil taking center stage in an RPG "for the first time"
Their is another school of people who claim that Ultima 7 is capable of being "your only Ultima" and it has some things that are overwhelmingly cool and will make you want to rethink how RPGs should be designed - eg the NPC schedule, and the persistence of its world etc - I do think that you should be warned that U7 combat is basically Goldbox gone sexual and wrong in every way - But the rest of the game DOES make it worth it (it's a more extreme version of Fallout 1&2's famous weakness in the combat area in exchange for strengths in most other areas)