You could have Otto III succeding with his universal empire scheme, Poland being turned into a vassal after the pagan uprising, sons of Bolesław the Wrymouth forced to join the empire, Luxemburgs winning the reunification of Poland in early 14c. or Jadwiga being better with the axe and succeeding in eloping with Wilhelm (at this point it would be rather difficult imo).
I would also recommend a POD in the 10. or 11. or 12. century. Later on, it would probably become more and more difficult.
The following thoughts are not exact but some rough and uninformed thoughts.
My rough and uninformed idea assumes 2. stages.
Stage 1:
It would be realistic to assume, that Poland could become a part of the HRE as a young kingdom when the Ottonian Dynastie ruled over the HRE.
This "integration" would be a gradual process, so at the beginning just some pledges/oaths to the Emperor. This could be increased, if the Emperor/king of the HRE travels more often into Poland and does some stuff (like some wars against neighbours and supporting the church in the area). We also need more influence/integration into the HRE in church matters. The first polish Archdiocese in Gnesen was founded by Emperor Otto III. for the duke Bolesław I. .
It would be important, if Poland would be a duchy or a kingdom, if you want a closer integration, i recommend having Poland as a duchy for long enough time, that it is considered to be an integral part of the HRE.
It is difficult to determine which amount of integration into the HRE means, that a territory is clearly part of it, there are in OTL various examples for grey areas which are somwhat inside/outside the empire, while the imperial ideology is universal.
Stage 2: (One/two or several centuries after stage 1)
After we established some links/connections between Poland and the HRE, we need some dynasty which controls Poland or large parts of Poland and some territories in other parts of the HRE and is elected as king, and later crowned as emperor of the HRE. Such a dynasty could appoint local ("Polish" -> but i doubt, that there would be a polish identity at that point) churchmen as bishops in other part of the empire, they would travel frequently through Poland, and use it as power base. If we have a tradition in a famous monastery/church/cathedral, that several "Polish" emperors are buried here, local elites could identify with the empire.
(Scenarios in which Poland disintegrates inside the HRE into several duchies are possible/interesting, but your goal is a united Poland inside the HRE, it is therefore necessary, that the ruling dynasty of Poland is powerful enough not to lose control against local nobles/bishops or towns)
Some Effects of a closer integration of Poland into the HRE:
Many things in the Baltic Sea (for example the Teutonic Order) are butterflied away.
I assume, that this would delay the formation of German or Polish identities. It is likely that the ATL-HRE would become more and more anachronistic compared to the neighboring kingdoms (especially France and England) (as in OTL). But as long as Poland and Bohemia are part of it, it would be far more difficult to speak about a "Holy Roman Empire of the
German Nation". We could probably see several strong regional identities inside this Alt-HRE.
I assume, that there would be, like in OTL some form of "Ostsiedlung" (Eastward migration of German speaking populations in the middle ages), but transformed by butterflies. A Polish ruler could profit from encouraging migrants. (Political Boundaries later on wouldn't overlap with ethnic/language areas)
Some form of christianization would probably occur for the Baltic people, like the Prussians and Lithuanians. Maybe some of these territories become part of the HRE too.
Poland would be definitely an important player inside the HRE, and large enough to act independently from it, even if ATL-Poland would be smaller than OTL-Poland.
If there is later on some formalized circle of electors (as in OTL, but in OTL several centuries after the POD), it would be difficult to deny Poland this position.
If the emperor controls Poland, he could try to influence areas next to Poland. If the emperor does not control Poland, good relations to the ruler of Poland are necessary.
If there are many emperors with control over Poland, the HRE could loose control over Italy earlier than OTL, since such Emperors could focus more on Eastern Europe.
If there are few emperors with control over Poland, the non-poland-controlling emperors could focus even more on Italy, since it would be dangerous to confront/hurt the interests of the ruler of Poland. If you want to reform the Empire, support from Poland is necessary. But if Poland would help defending the Empire, the Empire could maybe more likely survive (because of more resources/troops) against foreign threats. Maybe the coordination against threats like in OTL Turks or Napoleon could be better than OTL. But on the other hand, the Reichsarmee (Army of the HRE, or the troops the various principalities contributed in case of foreign threats) wasn't efficient in OTL, and we would probably have similar institutions in ATL.
The internal structure of the HRE wouldn't be much different than OTL in these scenarios.
The Imperial Ideology would probably incorporate Sclavinia (or the Slavs) (the first of the four women in the picture below, which explains imperial ideology well without much words, the others are Germania, Gallia and Roma) more. This would further strengthen the universal character of the HRE. An emperor could try to bring other kingdoms/duchies under his authority. This could result in wars etc. . But these trials would be most likely unsuccessful or result only in formal acceptance of the superiority of the emperor. (An HRE with Poland is even more overstretched than the OTL-HRE).
Things to consider:
If the emperor has problems with the pope, the pope could convince the Polish ruler to become anti-king.
If there is aggression from some form of ATL-Burgundy or ATL-France, it would be not always in the Polish interest to support imperial resistance against these western neighbors.
If an ATL-Reformation lead to Civil War inside the Empire, Poland would be an important participant.
If there are ATL-Crusades, the ATL-Emperor could instead fight in Eastern Europe against Baltic Pagans, which could reduce the probability of imperial participation.
What about Mongols invading the HRE-Poland (or butterflies)