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I have a Basilica Adminstratum, Manufactorum. etc.. assembled and painted.

Looking for tips or guides on how to base them nicely.

Medium density fiberboard, or MDF for short, is the best for basing most terrain. It is pretty porous so glue and paint stick to it really well. Slap the plastic kits onto the MDF, glue some sand and rubble and paint that stuff up! It really looks fabulous.

I would use MDF, as well. One word of warning, though - practical basing thicknesses (I think 1/8" is perfect for most tasks) are liable to warp if you're careless, as the usual glues (namely PVA, as either white or wood glue) shrink as they dry. Either go light on the PVA and clamp/weight the piece down while it dries or use an adhesive that doesn't shrink (2-part epoxies, silicone sealant, CA glue, etc.). If you avoid that one common pitfall, the stuff is great - good balance of cost, weight, rigidity, workability, and resilience.

I use 6mm MDF, and it is yet to warp after being given a big helping of PVA, so that seems a good thickness.

I just realised you're in the U.S. and I'm not sure what the Imperial is for 6mm. It's apparently a "Little less than 1/4"
Not sure how useful any of that is.

If you spray a layer of primer to your board before any glue or paint, it will help it resist warping.