Don't feel stupid, we all start where you are now.
The first thing to do is to look at the information needed in the equipment profile and collect that data. Since you have used your equipment before, if you have measured the volumes pre- and post boil then you should have a pretty good indication of boil off rate. If not, then you can take a typical volume you would start off with (measured) and boil it for 15 minutes and measure how much you have left. Multiply this by 4 and you have an approximation of your hourly boil off rate. With all measurements, the more accurate you are, the better off your results will be. I recommend making a dip stick for your kettle marked off in liter or half liter increments for measuring your volumes.
Now, there are several ways for setting up your system for maximum wort out. My first go-to would be to target around 8 liter batches. Assuming about a 2 to 3 liter per hour boil off rate, you can just barely fit the pre-boil volume into the kettle you have, but you may be able to limit how much you need to top off in the fermenter. I would plan on maximizing the amount of strike water and wort to come close to filling the kettle you have and then batch sparge with either hot or cold water in another pot (which can be of lesser capacity) or even a plastic bucket. At the end of the mash, take your bag of grains out of the kettle and transfer it over to the pot or bucket you will sparge in. Now pour in the amount of sparge water to make up the difference in volume from your wort left in the kettle to the desired boil off volume. Open up the bag and stir the grains to extract more of the sugars and then pull up the bag, let it drain, and then pour the collected sparge wort into the boil kettle. Doing this method will improve your efficiency, while trying to double the grains will decrease you total efficiency.
The other way to tackle this is to take some of the base grain out and replace it with DME or LME dissolved in your top off water and boiled briefly to sanitize it. You can make this up ahead of time, chill it and have it in your fermenter while brewing the rest of your batch. This is essentially the same as making two mashes to fill the fermenter, a very common practice in commercial brewing. A variant of this would be to brew your recipe twice to fill the fermenter.
I fully understand the desire to avoid spending money on more equipment, but if you want to really do a full volume BIAB you will need to have a kettle at least 2x your batch size. I tried both of the approaches above with an 8 liter stock pot for a few months before picking up a 21 liter stock pot for less than $60 US. That allowed me to brew a full volume BIAB and get 10 liters into the fermenter. 10 liters yields me a couple of bottles more than a case every time I brew. Like you, I like the variation so brewing more often and stocking up gives me a good selection to choose from throughout the year.
