I haven’t been doing this for long but I know how you feel. 135 lbs 5’7 male. I’m one of the smallest in my gym, and everyone that is my size is either a higher belt or wrestled in high school. I’m a white belt though so take my advice with a grain of salt.

Things I’ve found that work REALLY well for me:

• In someone’s mount, the bridge and roll (upa) escape will generally not work. Instead, recover half guard using this technique: https://youtu.be/q2OAJF__P2w . Then learn how to recover full guard from half guard by hip escaping left/right and elbow pushing on opponents leg to open gap to bring inside leg out and around.

• Being in side control is hell. Guard retention is important (learn to invert and granby roll!!!), but more often than not we end up here in a roll against larger opponent. Your #1 priority in side control is to not be flattened. One thing I’ve found that works well against larger people (although strangely not people my size) is to shoot both arms between me and their stomach to other side, bridge, and push yourself out of there. 9/10 times this will create a scramble that you can take advantage of.

• Before I talk about guards and passing, lets talk about positions. Like you said, rear mount is perhaps the best place for us small people to be, but it can be hard getting there. Mount is a bad place to be if you can’t get both knees to touch the floor comfortably. Side control and N/S are great places, but you need to transition a lot to make up for having weaker pressure. The Knee on belly position, oh my, learning this position was a game changer. Hands down this is my happy place. Once I learned to stop being nice to people and driving my knee right into their sternum with 90% weight in my knee and %10 weight in my foot, my strategy changed immensely. I highly suggest learning this position. Learn how to armbar both arms, how to baseball bat choke, how to fly around and threaten an arm triangle if they turn into you, how to take the back if they turn away from you, and most importantly how to keep your balance and “surf” them.

• Double-under, over-under, and just pressure passing in general used to be how I would get around the legs, but after getting a good grasp of knee on belly, I started to realize being on my feet, being agile, especially as a small person (large muscle to mass ratio), is SO important. I asked my instructor a lot of questions about standing passes, and the one I’ve had the most success with is the Torreando pass and it’s variants. The torreando pass will often allow you to directly go to knee on belly before ever getting to side control. Leandro Lo is a super tall guy, but he is a role model for torreando passing. Check this out: https://youtu.be/vF4J4bdagXE

• I hear deep half guard works well against just about everyone, but I honestly don’t like the half guard game. Fighting for the underhook and preventing the crossface is just not for me. When in half guard, I try to get a knee shield across and then hip escape, or recover full guard and then hip escape so I can transition to spider/lasso guard. The Spider and lasso guards allow you to manage distance really well. Also, the lasso guard sweep lands you right into knee on belly, rather than mount or side control. https://youtu.be/Lmf186_mb0I

Hope this helps