Jcan, sounds like we can welcome another sailor to the clan! (NB you don't have to be any good to join the clan; you just have to have done it!!).
Your learnings are almost spot on: 1. these boats are tippy - with time you will develop an appreciation of how far you can push it in a given wind-strength and fast reactions - until then expect the occasional unexpected bath
; 2. Sailing upwind puts more pressure into the sail (because you are sailing into, not with, the breeze, so the wind strength is increased by your own motion) and in stronger winds it can be difficult to sail upwind without sideslipping (being pushed sideways) if you have no keel or daggerboard - the fins do have some effect but not a huge amount - so best to experiment with sailing upwind in lighter winds so that you can get a good feel for what the boat is capable of: it will go upwind but not to anything like the same extent as a deep-keeled racing yacht.
The thing you have yet to learn relates to your fishing kit - it is a hard lesson and it goes like this: if it ain't tied on sooner or later you are going to lose it
. Based on my experience I strongly recommend that before you start sailing and carrying your fishing kit (let alone sailing and fishing) you work out a secure way of attaching everything to your boat so that when you turn turtle (note I do not say "if") you don't lose your valuable stuff. It is also worth considering securing your drive - even when you think it is securely locked into the drivewell inversion, even temporary, can do funny things with physics and your drive is too expensive to lose!
Personally I use strong paracord tied to the various padeyes on the boat with a small stainless steel springclip attached to the business end; these then clip onto loops of line on all my fishing rods so that I can switch rods between hands and holders quickly and easily without having the rod encumbered with a darn great length of line and a clip all the time (I can also have more rods than leashes and swap rods around between the rod rack and the kayak at no more cost than that of an additional loop of line on all my rods). The loop of line on the rods is set up the same way - in a short length of light strong line tie a loop that will fit on the palm of your hand (I use a simple simon knot - a reef knot with an extra round turn - much more secure than the reef knot - but a reef knot could be be satisfactory) ; twist it to make a figure 8, fold in two and slide the two loops up the rod handle from the base to the reel seat, position the cross of the figure 8 on the opposite side of the rod to the reel seat, seat your reel in the reel seat with one loop above it and one loop below it - now you have a loop of lie on your rod in the form of a figure 8 which (if it is small enough) won't interfere with your fishing and won't slide over the reel and fall off. Clip your security leashes (one on every rod holder position) to this loop and Davy Jones won't claim your rod & reel when you turn over.
There are plenty of other learnings on this site - I recommend ball bearing blocks to take the friction out of the sheet as an aid to avoiding wind-blown capsizes, a roller furler (the new Hobie one has to be the first choice option) and a boom batten for downwind stability. These three relatively cheap mods will significantly enhance your sailing experience from day 1. Later on you might want to think about a clam cleat to belay the sheet and give your hand a rest. All are discussed in this forum if you do a bit of searching.
Hope this helps and enjoy !