Technique will make the biggest difference.
If you are mainly a cyclist you will need to work on your upper body strength, all in the shoulders for front crawl.
Tumble turns will save you 2 seconds a length, plus making the most of your push off, you should go 5m before reaching the surface at each end. Reaching the surface when your push off speed matches your swimming speed.
I’d swim 100m intervals on 1:45 including rest time in sets, if you can’t hack it then 1:50, higher etc. Once you can do 6 or so in a row, start increasing the pace on future training sessions.
I am a better swimmer than bike rider by a long way.
Looking at Triathlons, they are much more bike/run balanced. For example, the local club does: 300m Swim, 17km Ride and 2.5km Run. Now a 300m swim I could do in perhaps 3:30 pushing hard. But then you are looking at half an hour on the bike, I would perhaps lose 10 seconds a km over a better rider who was only an average swimmer that I could only expect to gain 60-90 seconds over on the swim.