I sailed an original DS for some years as a kid. They ain't particularly weatherly, being about half as wide as they are long and very flat-bottomed. But you should still be able to get higher than 80 degrees true.
You'll need a compass on board. Doesn't even need to be calibrated, especially. Your goal is to determine your tacking angles. If you are sailing 130 degrees on one tack, as high as you can point before your jib luff breaks, then you tack and are sailing 235 degrees on the opposite tack, that's 105* between tacks, or 52.5* to the true wind. That's probably the best you can hope for on a DS with old sails.
Now you have something useful to compare to your GPS track. If your compass says you are tacking thru 105 degrees, but your GPS shows your angles over ground to be 160 degrees, one of two things is happening: either you are fighting adverse currents, or you are making bodacious leeway. Not much you can do about currents, except milk the eddies, the shoreline, and the shallows.
Leeway you can do something about. Try to sail the boat as flat as possible; the DS has really soft chines, so heeling doesn't help the hull bite the water any. You want the centerboard as vertical as possible. Check the CB and rudder & see if they are clean, undamaged, and not wobbling all over. Trim the jib as close to the centerline as possible while still keeping some draft in it. THEN trim the main. Just sheet in until the luff is barely soft. You should see a hint of backwind-ing, just behind the mast. If the main is board flat, it's probably oversheeted.
Trim the jib in tight, sail as high as you can go before the sail luffs, & that's your heading. Once you have that, trim the main to the edge of luffing or just barely luffing. Oversheeting the main will stall the boat and is the most frequent cause of excessive leeway.
Light winds will hurt your pointing ability, because you need more draft to make the boat go, and that widens your attack angle. Better to foot off & accept fatter tacks. Heavy winds will hurt your pointing ability, as the boat heels, draft in the sails moves back, and you are forced to crack the sheets to spill excess wind. Every boat has its sweet spot; for dinghies and light keel boats, that can be pretty narrow. Our SJ21 likes the wind between 8 and 14 kts true and needs heel angles between 10 and 15 degrees, or it won't point worth a damn. We can tack thru 80 degrees in 10 kts if we sit one person on the low side.
If it is light, or really windy, or there's any chop, we're lucky to get 100 degrees.
Higher boat speed (relative to true wind speed) hurts
pointing ability, as the apparent wind shifts onto your nose. Many performance multihulls can't point above 55 degrees TWA cuz boat speed is 2x wind speed! You'll learn to fall off in the lulls (header), but you can point higher during puffs (lift). Chop will kill your pointing ability dead
in this boat. Keep experimenting & let us know what you find out.