I would upgrade today. You are basically max TH8. At early TH9 you are likely to want some cheap upgrades and will put a builder on a collector rather than have them idle. Plus TH9 has more builder time than lab time, so some carryover lab from TH8 is useful (Th8 has more lab than builder, balance the two by upgrading before lab done)
You will only have issues at a given TH level if
- Farming troops are more than 1 TH level behind. Ideally at your TH level, but ok to be 1 behind, but not 2
- Resources in Storages/Collectors - If you don't spend them, someone will take them, so spend them.
- Serious about Clan Wars - You need to find an accepting clan and you'll be fine here too if you have the troops/skills.
Farming troops can be as simple as barch or it can expand to cover gibam+wb+spells. Your troops are already the minimum level for TH9 (TH8 on barch + minion is good enough), so given your max defense/wall status, I'd upgrade today.
It doesn't matter if you're inactive. As long as you farm in Silver inactivity is not a big deal (you don't have to maintain league) and if you spend resources, you won't lose them to anyone. Spent resources can be raided for 0%.
