I would take advantage of the transfer process, unless you have only a handful of members. I don't think it would be easy to replicate all the features of the transfer by hand.
As for delays in messages, that was happening yesterday afternoon due only to a freak occurrence. Someone sent a couple of GINORMOUS messages through the system - I know because I noticed the delay and messaged Mark, and he immediately messaged me back - on a Saturday! - and told me what was up. These kinds of issues are so rare in groups.io that I would say that they just don't happen. It is the polar opposite of yahoo here. OTOH I don't know how large your attachments are. Perhaps if they're really huge, it's causing a problem. Mark will let you know what kinds of sizes might cause an issue, if any.
If you want to pay, you can upgrade your group to "premium" for $10/month (or, I think, $100/year). I highly recommend this because it gives you a couple of additional capabilities in addition to just increasing your allowable storage.
Congrats on the decision. Y
Messages are the sole opinion of the author, especially the fishy ones.
I wish I could shut up, but I can't, and I won't. - Desmond Tutu