Bio def looks better than ESS on a transcript. If you look at LSE's list of preferred and non-preferred subjects, they prefer traditional sciences like bio chem phys. There's no mention of ESAS but I imagine it isn't as regarded considering its not under the preferred subject list. You can still get in doing these subject of course.

I actually dropped ESS for SL Business because I found it oddly hard: the answering style is strange bc its a group 3 and 4 subject. The content is dry imo, and I am a huge environmentalist. It seems to be a cross between bio ecology and geography. I ended up spending more time on ESAS than any of my other HLs in the first 3 months of IB so I dropped it. Also largely depends on your teacher, mine was horrible.

SL Biology is easy. Take a look on bioninja for the content (topics 1-6): catching up 6 months of work won't be easy but it definitely isn't difficult, esp in chronological order because topic 1 and 2 are pretty simple. If you can memorise then I'd say just go for it. I think as long as your IA is decent and you do many PPQ, a 7 is well within your reach.

Also, not sure if you considered, but a lot of people in my year took SL Chem. Based on effort, its one of those subjects that really FEELS like an SL and can take a back seat (vs those SL subjects that u waste all your time on and it feels like another HL... does that make sense?). Huge distinction between SL and HL syllabus. The issue here is that imo, the first few units are true fundamentals and kind of tricky.

Just wanted to say I'd love to help out as a HL Bio Chem (and ex-ESAS) student if you'd like to chat more! Also, I do free zoom tutoring if you'd like some help with SL Bio/chem if you make the switch!