If you’re posting relevant, quality & especially original content then it’s not spam. If you crosspost it to a bunch of communities on the same server that’s a little spammy, so maybe don’t do that, but I don’t think it’s spammy to crosspost to similar communities on other servers.

Also consider defining the scope of your new community differently. It might be the same topic as another community, but is it more focused? more nuanced? what’s the niche you want to fill that the larger community doesn’t, or doesn’t dive deeply enough into?

And finally manage your expectations. You might have to be the sole contributor for a year or more before you start seeing active participation from other users.