Martin Sellner really has the best position when it comes to realising our political aims. We're opposed to the great replacement and the policy positions to do that are simple. Our ultimate goal is remigration, that's more complicated but can be done even under the political norms of our time.
We have to work incrementally when it comes to policy, this idea that there's a swing-back in politics is ahistorical, losing just means losing and it's bad. Winning even small victories is essential as it means your enemy is gradually becoming unable to stop your will and emboldens your supporters to demand more.
We need someone in power to first stop mass migration and then start the process of remigration. The first goal is achievable by a civic nationalist party. The second process can begin gradually under a civic nationalist government.
A significant amount of immigrants that enter our country leave after a short stay and if they were simply stopped from entering altogether then remigration would already be in progress in a small way. As recent immigrants leave.
Around 50% of British-Pakistani marriages are to Pakistani's from aboard. This dynamic is present in practically all immigrant communities though less so than Pakistanis & Bangladeshis. Simply making the trafficking of wives and husbands from abroad for British resident minorities would force many to go home for marriage sake and prevent the making of highly fertile recent immigrant families. Then we can implement hostile legislation like banning the burka's in schools, and then in government offices and eventually on the streets as it was done in France (2004), Bulgaria (2016), Austria (2019) and Denmark (2018). We can then expand to the Hijab, as is being done in Austria. Then prohibit the building of Mosques, as is done in Greece (not a single legal Mosque has been built in Greece in 200 years). Once the building of new mosques is banned or made unfeasible we can use anti-terror and anti-organised crime legislation to start shutting them down, as France has done to literally hundreds of mosques with little opposition since 2017.
Simply getting them riled up and seeing our country as a hostile place would do wonders to create voluntary remigration.
All of this is achievable under a mild nationalist government and has been done in nations WITHOUT nationalists in charge.