I'm sorry that the DJV180 wasn't suitable for your task. As I said earlier my experience of the DJV180 is that it is a solid tool, but in terms of straightness of cut, unless you increase you budget to considerably above your original figure of £200 and go for either a Bosch GST160 (which features a secondary guide system) or better still a Mafell P1cc (which features a heavy, oversized shaft) you'll never achieve anything approaching a truly straight cut with a jig saw, and even then... But I do seem to recall also saying that jigsaws are NOT truly accurate devices, and most regular jigsaw users, I'm sure, would concur.
As you have surmised though, not all blades are equal. If you need a blade that cuts cleanly and consistently, you need to choose a blade which is specifically suited to the task at hand, from a "quality" manufacturer. In my own experience that boils down to blades from Bosch and Metabo (both of whom source their blades from Scintilla in Switzerland - a Bosch subsidiary, and the inventor of the jigsaw back in the late 1940s), Wilpu (German), Starrett (American, but also manufacture at Jedburgh in the UK) and Lenox (American). I've tried others but never been that happy with the results.
For the straightest and cleanest cuts you also need to turn the oscillation (used to gain speed) down to zero.
Another factor with straightness of cut is that of blade use. If you are constantly sawing curves jigsaw blades (and for that matter scroll saw blades and bandsaw blades) have a tendency to take on a set to one side or the other. Saw a number of tight anti-clockwise circles with a new blade in thick stuff like a kitchen worktop, then try making a straight cut with the same blade in the same material. It's an interesting exercise (in frustration)
Lastly, as has been frequently stated on this forum and elsewhere, cordless tools do not have anywhere near the same power output as equivalent capacity corded tools. It stands to reason that an 18 volt brushed cordless jigsaw will only have the same power as something like a 200 to 300 watt corded tool, if that. This means that battery tools are a lot more sensitive to blade thickness and sharpness because they are relatively so low powered.
An example of this: I am currently fitting a lot of cement fibreboard floor insulation (about 3,000 square feet on the last week), which involves making large numbers of saw and jigsaw cuts where we need to fit around stair openings, queen posts, steel stay rods, ventilation ducts, electrical wires, soil stacks, etc. I've ended up with a corded (Bosch) plunge/rail saw being used for the straight cuts rather than my 36 volt cordless (Makita) plunge/rail saw which used a lot of batteries cutting the boards. The vacuum extractor is a 36 volt cordless/110 volt mains hybrid (Makita) - but it is being used on 110 volt because on battery power it doesn't clear the dust sufficiently well, and battery life is so lpoor when used heavily. Small cut outs are being dealt with by a Bosch GST150BCE 110 volt jigsaw with third party TCT blades, because I find my Makita DJV182 (brushless, cordless) is a bit lacking in power for this material. In fact the only cordless tool I am using is a Makita DHP481 combi drill wich is used to drive a couple of different sized TCT grit hole saws. This cordless kit had been good enough to install about 35,000 to 40,000 square feet of 18mm hardwood sub,- flooring and 9mm birch ply diaphragm flooring over the last 3-1/2 years requiring many thousands of crosscuts, trim cuts and squaring cuts so it isn't incapable.
My other installation team, doing roughly the same job but with DW brushless cordless tools similarly found that their cordless tools were struggling a bit with the material, and so switched to corded kit as well.
Put simply, even with the right blades there are jobs cordless tools don't do well on because they lack the necessary power and where used tooling choice and sharpness is absolutely crucial (a true case of NOT "anything goes")
So what material are you sawing, what blades are you using, how thick is it, and what radius curves are you cutting? Maybe someone could suggest a more suitable blade for the task