Well, some clades can surely be proto-slavic/proto-south Slavic. More probably the former as opposed to the latter. However, alot of it rests on minimal ancient DNA evidence with modern distributions guiding us. Which can be problematic when taking into account industrialization, and the advent of nationalism, and socio-cultural borders.
Considering how much the ethnic landscape can change in a few centuries, there's still a gap over a millennium wide between the only 2 ancient M458/L1029 samples we have and the post migration Viking, Slavic and German samples we have from the 8th-14th centuries.
Just spit balling here, but based on what I have seen it appears out of 18% R1a on poreklo, only about 5% of that is M458. Most of which is YP417 under L1029, and more specifically subclade YP6047 which is found in Croats and Bosnians as well. Though it has a dominant representation amongst Serbs. This and R1a-M458>A11460(a subclade closer to the root of M458) has Serbs forming a clade(1400ybp) and a Pole that is basal who is 2800ybp with them. One of these is a Serb fom Kosova.
Among Bulgarians, they are 60/40(Z280>M458). Their M458 is dominantly L1029. Within L1029, not counting any less common clusters, YP417 and YP263 dominate, with YP417 being higher than YP263. I'm sure they have L260 as well considering it pops up in Albanians. Though L260 seems most rare among Balkan M458 branches. I may be wrong but I think it showed up in Shkoder and some in the South. We also Plan/Pult clans who are also R1a-M417. Most likely CTS3402 under Z280. Not yet confirmed. Have a couple in Mat and one in Kercove that is likely basal Y33* under Z280, which is quite interesting given the tmrxa is Iron Age.
Recently an Albanian from North Dibra was A11460 which is common in Serbs. Though it remains to be seen if hes close to those Serbs or form his own branch. Distribution wise most R1a in South East Slavs has a 60/40(Z280>M458). Among South West Slavs Z280 is more dominant. M458 peaks in Bulgarians, more especially central Bulgaria.
I really don't think L1029 has much to do with Thracians. I wouldn't call it Polish because they didn't even existed when L1029 formed or even when the most recent ancestor lived.
I think the most you can connect L1029 with Thracians, is potentially East La Tene outlier who brought it into Thracians(now that we have it in La Tene coming up). I still think East La Tene was dominant in R1b though so it would have probably still been more common than L1029, even if one managed to go in that direction.
And even still , I am of the opinion that L1029 became assimilated into Proto-Slavs so even if its pre-Slavic origin was East La Tene or something else; its movement in late antiquity and early medieval is most probably associated with Slavic migrations, Viking Culture and maybe Ostrogoths. Central European Celts dissappear from mention by the end of the IA.
I think if any East Germanic or East Celtic peoples are affiliated with M458 or Z280, it would be Bastarnae. There never was a firm agreement on whether they were Germanic, Celtic or Proto-Slavic. They were the most Eastern ones anyways. Theres also the Neuri who are believed to be Proto-Slavic but classified as either Celtic or Baltic by some.
Z280 is however the original Balto-Slavic line as it united Baltic and Slavic branches of R1a. M458 in Balts has no diversity and only dates to late antiquity and early medieval. Most basal M458 branches appear in Poland so I believe M458 may have originated there or around Czechia. Though its just guesswork based on what we have so far.
Here's a list of all the M458/L1029 samples. Alot of the Viking samples were "Polish-like". Though on PCA they appeared Baltic shifted. Viking was a profession afterall and Baltic Vikings were some of the last pagans practicing raiding.
List:
-760 BCE, Late Hallstatt, Singen, Swabia, Germany
-Iron Age La Tene, Bohemia, Czechia(in peer review)
-800 AD, Viking, Galgedil, Funen, Denmark
-900 AD, Viking, Sigtuna, Uppland, Sweden
-900 AD, Viking, Kurewanicha, Zalesye, Russia
-900 AD, Viking, Kopparsvik, Gotland, Sweden
-900 AD, Viking, Kopparsvik, Gotland, Sweden
-900 AD, Viking, Frojel, Gotland, Sweden
-900 AD, Viking, Sandomierz, Lesser Poland, Poland
-900 AD, Viking, Ładoga, Gardariki, Russia
-900 AD, Viking, Skara, Western Götaland, Sweden
-973 AD, Viking, Cedynia, Lebusland, Poland.
-975 AD, Viking, Ciepłe, Pomerania, Poland(2 samples)
-1060 AD, German admixed Slavs?, Wüstung, Saxom Anhalt, Germany(2 samples)
-1158 AD, German, Wüstung, Sax.-Anhalt, Germany
-1100 AD, Usedom, Mecklenburg-V., Germany
-1200 AD, Rus Viking, Łuck, Wolhynia, Ukraine
1400 AD, German-Dutch, Oldenzaal, Overijssel, Netherlands.
1560 AD, Muskovite, Radonież, Moscow, Russia., Sweden.
973 AD, Viking, Cedynia, Lebusland, Poland.
975 AD, Viking, Ciepłe, Pomerania, Poland, twice.
1060 AD, German? Slavic?, Wüstung, Sax.-Anhalt, Germany, twice.
1158 AD, German, Wüstung, Sax.-Anhalt, Germany.
1100 AD, Slavic (postViking?), Usedom, Mecklenburg-V., Germany.
1200 AD, Norse (sic!), Łuck, Wolhynia, Ukraine.
1400 AD, German-Dutch, Oldenzaal, Overijssel, Netherlands.
1560 AD, Muskovite, Radonież, Moscow, Russia
M458 heatmap(may be out of date but includes all branches):