@ggalmazor and I are picking back up some analysis of your forms. From the first post, it sounded like child_vaccination_VOL_tool_v12 was the form giving issues but follow-up posts made me less certain. Could you please confirm? What's the relationship between the VOL and SUP variants?
If you have a rough estimated range, that'd be helpful (e.g. 20-40 households, 2-5 children per form).
Some services that couldn't run without connectivity might be able to run so there could be a slight difference but it shouldn't be much.
Please note that changes to Android will mean that the Play Store version of Collect will no longer be compatible with OpenMapKit by August. See Collect will need to stop using /sdcard/odk for files for more. If OMK is critical for you, please make sure you communicate that with the Open Map Kit team. It sounds like the most important functionality for you is being able to select features, is that right? Do you need to natively produce OSM files? We'd like to bring at least some of the OMK functionality into Collect to make it more readily usable.
The RAM needs likely wouldn't be at the same time so it might not make a difference. Like you say, experimenting is going to be your best bet.
Given that you were able to complete your data collection with so few problems and that your form is quite small, I'm not convinced that there's a systematic RAM issue. I think what you're describing about making sure that the phones have received their updates and have sat for a while before fielding them sounds like a great idea. I don't believe there is a way to stop system work at a certain time but if you let the devices sit online for some hours, confirm that they are idle and then take them offline, I don't think there's much they could be doing.