SharePoint for Events management and travel booking

My project could be described as follows (see also the picture): 1. I regularly plan certain types of events (residential and online activities). 2. I want to change the existing manual process of delivering emails with word agendas and registration word forms attachments). I share these events directly with potential participants or experts but in most cases I share the events with one representative/delegate in one country. These delegates in their turn share the event as well and finally nominate/propose ranked participants from their own country units. 3. Once I (me or the course organizer) approve their proposals/nominations for participation in the event (usually a maximum of 2 per country unless more places are available) they become registered to the event. 4. Once the users are registered for the event, a travel booking process begins with the support of a travel agency for travel and accommodation (unless the event is online) by sharing with them certain fields of the participant (if required to travel). The cost of the travel or/and accommodation may be paid by us or not. 5. I also often send and receive requests from others for Missions abroad. Once approved I manage them through the travel booking process as well. I can implement a SharePoint implementation both on premises or in the Dynamics 365 cloud. I'm looking for a SharePoint App or site to manage these meetings, missions, training events and travel booking. It may involve internal (AD integrated) and external (by publishing all or some details of the event with an specific web link) users. It may require more than one approval workflow. One to approve the nominations to the events by the delegation's units in the countries and another one to approve the costs of the bookings of the registered attendees/participants. Ideally, this solution should be highly responsive, user friendly and mobile enabled. The more configurable and less code and platform dependent the better. It could be used for more than one or in different environment scenarios. Please feel free to make any inquiry to come with an approach.