Hi Phil and welcome!
Have you tried illustration board? It would not need to be gessoed.

The most stable are aluminium composite boards having none of the issues wood has.Very nature of wood is to expand/contract taking in/giving out moisture but it does so ineffectively if kept at wrong temperature or place(ie if piece is made in one place and then transported to say an exhibition by truck or sea in another place this can create horrendous changes in it also) or too much humidity warps etc.If you want to use wood keeping at it at correct temperature (also dureing any transportation) sealing it will help when you seal it it is stopping the natural resins from rise to surface is vital to aply some kind of seal to stop this a stain blocker either water based or Zinsser bin type product.(Never use a piece from a recycled wardrobe or something UNLESS you know for sure it has no varnishes on it etc ) is best to buy a piece that is not rough sawn scaffold plank type wood get as is said here poplar or mahogany etc sand very smooth.PVA stops foreign matter on paper or wood knots coming through it can act as a size before primer BUT NOT AS A PRIMER as does not give bond well it will stop wood absorb stuff from the paper on top and use only pva artsits grade not diy stuff. You could do belt and brace and do all this and on a cradeled board.Also in the past I have recycled many pieces of wood for such project thinking was doing a good thing BUT have learnt lesson regereted is as found you are forever playing a detective as to why paint layers do not look good in some way or another as whatever was put on the wood before even after sanding and striping sealing primer etc etc etc( varnishes stains or old rabit skin glue is soaked deep within the wood) and come up in work many layers on so also is case for pallet scaffold type wood is never get smoothe and see imperfections many layers on :/

ACM panels are great for non warping supports for anything, but they really shine with with water based stuff. They are easy to cut in straight lines with nothing more than a craft knife and a metal straight edge, but not so easy to cut into curved or irregular shapes, unless you have a mini router or water jet cutter. 3mm is the thickness I use the most. 2mm is nice for small and light stuff but bends quite easily, 4mm is what I pick for larger than 24″ panels.

If you lightly sand the surface without breaking through the white polyester coating, then prime with XIM UMA bonder / primer, any type of paint or glue will adhere well.

This post gives quite a bit of information on various brands of ACM panels.

https://www.wetcanvas.com/forums/showthread.php?t=1466279

I use gator board. You can gesso it if you want to. Does not warp if wet. Comes in a variety of widths and can be easily cut to size. Very lightweight for framing.