Couple observations based on my experience (not necessarily about layout).
I did a curved layout with convex buttons because I wanted my cab to look modern. If doing it over, I would do the plain "straight 6" street fighter layout with
convex CONCAVE (corrected) buttons. On games with more than 3 buttons I find myself looking down at my hands to reposition my fingers because there is nothing to index or center on the convex buttons. I wanted the "4 in a row" for NEO GEO games (and my cab is NEO GEO themed), but found that I like using my thumb on the first button in the second row rather than reaching with a pinky or ring finger for that fourth button.
Some steam games like Castle Crashers require something like 8 buttons per player for all four players. One of the new Mortal Kombat games I installed required me to map every button on the controller before exiting the mapping screen and I had to jump a wire because I only had 7 buttons instead of 8.
If not worried about PC games, 6 is fine for P1&2. IIRC there are two or three games in MAME that used four buttons for P3 & 4. Most notably the dungeons and dragons games. IIRC Guilty Gear Isuka (demul emulator) uses 5 for player 3 & 4, but it's not worth adding a 5th button for IMO. If not worried about those, 3 is enough.
Robotron used 8 way sticks.
What to do about 4 way joysticks depends on whether it is the feel or just the function that is important to you.
You can get automatic 4/8 way switching joysticks (based on Sanwa JLW IIRC) from Ultimarc.
https://www.ultimarc.com/arcade-controls/joysticks/servostik/?gad_source=1&gclid=Cj0KCQiAyoi8BhDvARIsAO_CDsDQ1VTwb_L3fuNQ-DsTNRR9myU1pDlwAVJdf1cTZ-919pEXQshrauYaAgb9EALw_wcB They'll simplify the control panel, but won't have the stiffer centering grommet feel of a Pacman stick. I can't comment on how they feel as I haven't used them.
If you want the 90's Mortal Kombat/Street Figher/Side scrolling Beat 'em up feel, then IL Eurosticks are the way to go IMO, but I am not aware of any 4/8 way switching solutions for them. I'm using Sanwa JLFs, but it took like $200 worth of modifications to get them to feel right and some of the parts I used are no longer available. The big thing with JLFs is having to add a stiffer spring to make them feel more like a 90's joystick, but that causes some binding which requires a ton of lubing to overcome and wears parts out faster.
Playing driving games standing up sucks. Playing driving games without an analog gas pedal causes issues in some games like pole position. If you do not slowly apply the gas, the tires will spin and cost you enough time that you cannot progress in the game. There are options in MAME to convert the button press into a virtual analog signal that ramps up and down, but I'm yet to see someone implement it well enough to enjoy the games. Just use the spinners and enjoy some Sprint/Ivan Stewart's Offroad with them. You can get away with on/off gas in those games.
Most important thing is to heed the picture in my signature.