Keep in mind that outright parliamental majorities are unusual, except for Bavaria. Usually there will be coalition agreements between different parties.
The parties with a good chance to end up part of a coalition are the Linke, Grüne, SPD, FDP, and CDU/CSU (depending on where you live). The AfD has little chance to find partners or to win an outright majority, and all others are too small.
A small party might well have a good chance to find coalition partners, and if the issue is important to the small party they might try to preserve it during the negotiations. The same applies if none of the other parties has strong opinions on the issue.
That's assuming you get your position on your issue as an accepted part of the party platform. It will take a long time, and many allies, to get into the federal Wahlprogramm. It might be easier at the municipal level.
Keep in mind that even if your position is a good idea with no negative side effects, almost all policy takes money to implement. Money is limited, so supporting your issue will take support away from other issues. You will have to find allies.
If your specific issue is well aligned with one of the smaller parties, e.g. environmental issues with the Grüne, or tax reform with the FDP, then you will find plenty of other people where with strong opinions. Your issue might be reduced to half a sentence hidden in the party manifesto. If you go to a party which is not traditionally aligned with your issue, you might have a better chance to become the local expert.
But these tactical thoughts are almost secondary. None of the parties is just waiting for a guy who says "stop whatever you are doing, listen to me, this is important." If you want to influence policy, you have to "work your way up" from the local level, learn procedures, make alliances.
Visit the local chapter meetings of some parties you might want to join.
Tell them that you are making your mind up if you want to join that party or perhaps another one, and that you'd like to watch. The local meetings will be a mix of routine administrative issues and policy discussions. They might be working on a paper they're preparing for a regional party conference, or set the schedule for next year, or whatever. There may be some situations where they are not exactly happy about newcomers, but generally they'll let you in.
Find out if their local chapter is active or lazy, if you like the people, if you can imagine standing with them in the pouring rain and handing out leaflets.