Absolutely don't trust the lower limit. First, the temperature mentioned on these bags are measured for men. So if something is comfort temperature -9 for men, it is perhaps -1 for women.
Then, the temperature that they report on is based on very standard conditions. In real conditions, it is again worse. E.g. A -9 might be functioning like a -1.
I currently got a down sleeping bag with comfort temperature 7c, limit temperature 4c, from Sea to Summit, but it definitely is not comfort temperature of 7c. It is more of a 15c comfort.
Also, if you are not constantly hiking in freezing temperatures, there is no need to buy a huge sleeping bag. Worst case, you can rent one or buy a second hand, if it is about using it only a bunch of times.
I got a silk liner for 50€ from Decathlon, it is super small, and 120gr. Don't buy it with the hope to add warmth, it just keeps your sleeping bag clean.
I use the Spark 45 for summer bikepacking and it’s been great. But if you wanted to pack even smaller then I’d look at Sea to Summit’s fleece bag liners by themselves. I’d imagine one of those with possibly sleeping in light layers would be quite comfortable at 60F. I’ve also got their silk bag liner but not sure I’d use it by itself at those temps.
I have the Neve Waratah quilt(-2C version) and prefer it to my Sea to Summit Spark sleeping bag. I use it with the Therm-a-Rest NeoAir XTherm NXT sleeping mat. The mat is a R 7.3 so might be too warm for you but I’m a cold sleeper. However that being said I have used it in summer in the Blue Mountains and I didn’t overheat but I’m not sure how it would go if the weather was a lot warmer.
i feel you on the gear hunt for a trip that big. for those temps and your height, look at the Sea to Summit Spark SpI sleeping bag - packs tiny and goes to -1°C comfortably. pair it with the Therm-a-Rest NeoAir XTherm sleeping pad, that combo handles cold and wet really well. both are super compact for bikepacking and should fit your 187cm frame. the Spark comes in a long version too. i've used similar setups in norway and denmark in september and stayed warm enough
Take a real hard look at the Sea to Summit Sparc rated to 30f, also comes in a 45 and 15. Beautiful bag, stuffs to the size of a 1ltr water bottle. Fabric and down technology is,in my humble opinion, unmatched. I’ve been in the industry for many decades, owned and tested more bags than I can name. S2S bags are a well kept secret.
Sea To Summit makes, in my opinion, the best warmth to weight/ bulk equipment on the planet. It’s not budget, but it’s not the most expensive line out there and they do occasionally have sales. Take a hard look at the Sparc line of sleeping bags.
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