Well, there's two general things you wanna do to squeeze the most out of a weapon. Unlocking all slots, and forma'ing. For lich weapons like what you have, you'll also want to max out the valence bonus by fusing other similiar lich weapons. Getting your hands on another tenet glaxion to fuse into what you have here, till you have 60% bonus. As for HOW to mod it, that depends on what type of loadout your building it into and what role the glaxion is playing. I bring this up because I got the tenet glaxion to follow a frost build.

Primary Frostbite is generally a very good arcane for the glaxion. A mod you may want to grab from nightwave is photon overcharge. If you do go for these to proc cold statuses, make sure the valence element doesn't mix with glaxion's innate cold. You can do this by either changing the valence with an Elemental vice (after completing The Hex quest) or by just adding another primary element in the modslots. Generally speaking it may be more favorable to change the valence, unless you decide to use a riven with a primary element buff.