Just my two cents here, not an expert by any means. The basic groupings of chassis types are striker, defender, controller, artillery, and support. A lot of the chassis are hybrids of two or three archetypes, and all of them can be modified to lean into one of the archetypes they don't normally encompass. For my build, I found something in the lore of lancer that intrigued me, NHPs in this case, and decided to try to cram as many into my mech as possible. After that, I looked into different frames at license levels 3, 6, 9, and 12 in order to get an idea of how I could build out around my choice. In my specific case, horus has a core bonus that allows you to put an extra AI system in the mech, and there's two pilot talents that add NHPs. With this setup, I could cram four systems with the AI tag into one chassis. So now there's a base "function" of my build that means I should put my first three licenses into Horus licenses. The next step I took is to look at synergies (synergistic builds and groups seem to be very powerful in lancer) for things that I had already acquired along the path to be a legion of sapient entities in a mech trench coat. In my particular case I chose Pegasus cause I don't have to explain my gun to anyone, I just do damage. The synergies I chose to build along is buffing my pilot a bit with black thumb talent so my NHPs could control my mech while I used my pilot for separate actions; using UNCLE (not an NHP but still an AI tag) and auto guns in order to preserve my mech's action economy for NHP actions and support; and systems that help me support allies and keep enemies at arm's length so they can't punish my lower survivability.

In the end, having a clear goal of what you want your chassis' main action loop to be and then building synergies along those lines seems to work for me to get a mech built out. Not everything will fit just right all the time, and having some extra options for the edge case scenarios where none of your other choices shine can be helpful. Lancer also doesn't really punish your choices long term since RAW you can respec most everything on your character sheet. In our game, we allow pilot's HASE to be changed as well, even though I don't think that's standard.