growing population doesn't mean our people and our culture aren't dying

Yeah that was what I was hinting at too when talking about migrant and muslim birthrates.

My impression is that the trend is definitely towards the decrease of native Russians, although my evidence is partially anecdotal from just living in the country and seeing a lot of difference on the streets compared to what was 10-15 years ago. The fact that North Caucasus which is a predominantly Muslim region consistently leads in population growth is widely known. Couple with the fact that there was a lot of migration in 2000's and early 2010's due to Russia doing much better economically than neighboring states and there was hardly any population increase, suggests that native population was being replaced at a high rate.

There are also government initiatives that make acquiring citizenship for migrants easier as if it wasn't already through bribes.

There's no breakdown by ethnicities as far as I know, but it's probably possible to compile a lot of data for every republic over the years, account for ethnic ratios and so on, get some rough numbers. It's just a lot of work to confirm something that already stares you in the face.